Re: Why are we waiting?

We've got various facilities now for looking at LWLock waits, but I'd
like to have more information about the *reasons* for lock waits.

I know its possible to get backtraces in Dtrace at various tracepoints
but that can be fairly hard to interpret.

I'm thinking of adding an extra parameter onto every call to
LockBuffer() and LWLockAcquire() to explain the reason for the lock
request.

Then when we analyze lock waits we can apportion the lock wait reason
correctly and determine not just where the waits are happening, but
*why* the waits are happening. Activity Based Costing the beanies call
it.

This will be especially helpful with transitory events that might (or
might not) occur fairly frequently, yet the point of contention moves
around within the server. There's a few hotspot reasons that move
around, plus I'm certain there's one or two we have no idea about.

This won't help much for highly contended locks where a lock queue of
100 might have 99 fast lock holders and 1 slow one. But it will help for
the transitory locking where a lock is frequently not held, yet we want
to explain what was happening when the lock *was* held.

Reason would be an enum value and reporting would probably be covered by
LWLOCK_STATS.

--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com

Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> I'm thinking of adding an extra parameter onto every call to
> LockBuffer() and LWLockAcquire() to explain the reason for the lock
> request.

Maybe I'm missing something, but I don't see what this would buy us,
except for being able to track which call site resulted in a wait;
which can already be measured with dtrace, no?

I'm hesitant to decorate such widely-used functions with extra tracing
information. You'd be breaking every third-party module and pending
patch that uses either function, and imposing some performance penalty
(probably not much, but it's hard to be sure) into the indefinite
future, for a performance measurement need that might be fleeting.

Basically I'd rather try to attack the problem with dtrace ...

regards, tom lane

On Mon, 2008-02-04 at 17:06 -0500, Tom Lane wrote:

> Basically I'd rather try to attack the problem with dtrace ...

OK. I'll switch to Solaris. Or do you something I don't about dtrace on
linux?

--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com

Simon Riggs wrote:
> On Mon, 2008-02-04 at 17:06 -0500, Tom Lane wrote:
>
>> Basically I'd rather try to attack the problem with dtrace ...
>
> OK. I'll switch to Solaris. Or do you something I don't about dtrace on
> linux?

One idea would be to add new arguments to LWLockAcquire as you suggest,
but instead of modifying all call sites, decorate it with a macro that
passes __FILE__ and __LINE__ as the extra arguments. The good thing
about that is that it's a relatively small patch and you can easily
switch it on/off with a #ifdef. And there's no need to push for
inclusion of that into CVS, because it would be an easy patch to
maintain yourself.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com

On Tue, 2008-02-05 at 14:14 +0000, Heikki Linnakangas wrote:
> Simon Riggs wrote:
> > On Mon, 2008-02-04 at 17:06 -0500, Tom Lane wrote:
> >
> >> Basically I'd rather try to attack the problem with dtrace ...
> >
> > OK. I'll switch to Solaris. Or do you something I don't about dtrace on
> > linux?
>
> One idea would be to add new arguments to LWLockAcquire as you suggest,
> but instead of modifying all call sites, decorate it with a macro that
> passes __FILE__ and __LINE__ as the extra arguments. The good thing
> about that is that it's a relatively small patch and you can easily
> switch it on/off with a #ifdef. And there's no need to push for
> inclusion of that into CVS, because it would be an easy patch to
> maintain yourself.

Thanks for the idea. It had occurred to me to make a private patch, but
I prefer my patches to be open, so they're easier to discuss results
arising from them.

--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com

Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> On Mon, 2008-02-04 at 17:06 -0500, Tom Lane wrote:
>> Basically I'd rather try to attack the problem with dtrace ...

> OK. I'll switch to Solaris. Or do you something I don't about dtrace on
> linux?

Nope :-(. The SystemTap guys keep promising support for userspace
probes but it's not there yet.

regards, tom lane

On Mon, 2008-02-04 at 19:46, Simon Riggs wrote:
> We've got various facilities now for looking at LWLock waits, but I'd
> like to have more information about the *reasons* for lock waits.
>
> I know its possible to get backtraces in Dtrace at various tracepoints
> but that can be fairly hard to interpret.

Simon,

A couple of guys here at Sun have started looking at trying to improve
the scaling on Sun's T2000 Niagara servers (single core, 32 HW-threads).

We mostly use various DTrace scripts to monitor locking usage. We'd be
happy to share results, probes and DTrace scripts for monitoring if you
like.

So far we've found lock contention (especially for the ProcArrayLock) to
be the likely source for our inability to saturate the CPU with a TPC-C
like OLTP load with >1000 users.

The following shows average time spent in and waiting for exclusive
ProcArrayLock vs the time used in the transaction through commit, (i.e.,
up until the exclusive ProcArrayLock acquire to update the PGPROC
setting transaction no longer running):

# ./pg-lwlock-procarray.d $(pgrep -n postgres)

postgres`LWLockAcquire
postgres`ProcArrayEndTransaction+0x10
postgres`CommitTransaction+0xf0
postgres`CommitTransactionCommand+0x90
postgres`finish_xact_command+0x60
postgres`exec_execute_message+0x3b4
postgres`PostgresMain+0x13a0
postgres`BackendRun+0x27c
postgres`BackendStartup+0xe0
postgres`ServerLoop+0x1a0
postgres`PostmasterMain+0xcbc
postgres`main+0x1d8
postgres`_start+0x108
23
avg lwlock acquire service time [ns] 193051989
transaction-commit [count] 23
transaction-start [count] 23
avg transaction time [ns] 12763079

The stack trace shows that the only time the lock is acquired
exclusively is from the call to ProcArrayEndTransaction() in
CommitTransaction().

Also, an interesting observation is that the hot locks seem to have
changed from v8.2 to v8.3, making the ProcArrayLock more contended. See
the following outputs:

PostgreSQL 8.2 (32-bit):

-bash-3.00# ./825_lwlock_wait.d

Lock Id Mode Count
FirstLockMgrLock Shared 2
ProcArrayLock Shared 209
XidGenLock Exclusive 1030
XidGenLock Shared 1215
WALInsertLock Exclusive 3942
CLogControlLock Shared 4113
ProcArrayLock Exclusive 6929
WALWriteLock Exclusive 17155
CLogControlLock Exclusive 128182

Lock Id Combined Time (ns)
FirstLockMgrLock 24705
WALInsertLock 79644210
XidGenLock 179886846
ProcArrayLock 1486950738
WALWriteLock 18425400504
CLogControlLock 1507388036453

PostgreSQL 8.3 (64-bit):

-bash-3.00# ./83_lwlock_wait.d

Lock Id Mode Count
SInvalLock Exclusive 1
WALWriteLock Exclusive 1
SInvalLock Shared 20
CLogControlLock Shared 534
CLogControlLock Exclusive 845
XidGenLock Exclusive 1140
ProcArrayLock Shared 1821
WALInsertLock Exclusive 17043
ProcArrayLock Exclusive 49762

Lock Id Mode Combined Time (ns)
SInvalLock Exclusive 17216
SInvalLock Shared 531129
WALWriteLock Exclusive 2003064
CLogControlLock Shared 61325819
CLogControlLock Exclusive 73509195
XidGenLock Exclusive 929477929
WALInsertLock Exclusive 17941476941
ProcArrayLock Shared 31114676303
ProcArrayLock Exclusive 888356047549

Regards,
Staale Smedseng
Database Technology Group, Sun Microsystems

On Wed, 2008-02-06 at 15:30 +0100, Staale Smedseng wrote:
> On Mon, 2008-02-04 at 19:46, Simon Riggs wrote:
> > We've got various facilities now for looking at LWLock waits, but I'd
> > like to have more information about the *reasons* for lock waits.
> >
> > I know its possible to get backtraces in Dtrace at various tracepoints
> > but that can be fairly hard to interpret.

Thanks for this Staale.

> Lock Id Mode Count
> ProcArrayLock Shared 1821
> ProcArrayLock Exclusive 49762
>
> Lock Id Mode Combined Time (ns)
> ProcArrayLock Shared 31114676303
> ProcArrayLock Exclusive 888356047549

Which looks like a mean service time of 17ms for X lock waiters.

What is the frequency distribution of lock wait time on ProcArrayLock?
Does the distribution vary over time?

--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com

"Staale Smedseng" <Staale(dot)Smedseng(at)Sun(dot)COM> writes:

> The stack trace shows that the only time the lock is acquired
> exclusively is from the call to ProcArrayEndTransaction() in
> CommitTransaction().

I'm not sure but I think that's only true in 8.3. As I understood it in 8.2
transaction start also needed the exclusive lock.

> Also, an interesting observation is that the hot locks seem to have
> changed from v8.2 to v8.3, making the ProcArrayLock more contended. See
> the following outputs:
>
> PostgreSQL 8.2 (32-bit):
>...
> PostgreSQL 8.3 (64-bit):
>...

I'm not sure 32-bit and 64-bit cases are going to be directly comparable. We
could have a problem with cache line aliasing on only one or the other for
example.

But that is a pretty striking difference. Does the 8.3 run complete more
transactions in that time?

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Ask me about EnterpriseDB's 24x7 Postgres support!

> What is the frequency distribution of lock wait time on ProcArrayLock?

See below for wait time distributions for ProcArrayLock (both shared and
exclusive). The time measured is from entry into LWLockAcquire() until
return. I've recorded the same data in two different resolutions (ms,
and us for the lower part of the distribution). The DTrace script is at
the bottom.

These results are for 1000 TPC-C like clients, and measured over the
1000 PostgreSQL processes over a period of 10 seconds.

> Does the distribution vary over time?

Hmm... I will have to get back to you on that one.

Staale

CPU ID FUNCTION:NAME
6 71245 :tick-10sec

Total LW_EXCLUSIVE 25178
Total Transaction Starts 25679
Total LW_SHARED 107211
LW_SHARED [ms]
value ------------- Distribution ------------- count
< 0 | 0
0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 100565
10 | 1
20 | 0
30 | 0
40 | 0
50 | 0
60 | 0
70 | 6
80 | 304
90 |@ 1370
100 |@ 2685
110 |@ 1731
120 | 307
130 | 13
140 | 0
150 | 0
160 | 5
170 | 4
180 | 26
190 | 36
200 | 24
210 | 61
220 | 49
230 | 15
240 | 0
250 | 0
260 | 0
270 | 0
280 | 2
290 | 2
300 | 0
310 | 2
320 | 1
330 | 1
340 | 0
350 | 0
360 | 0
370 | 0
380 | 0
390 | 0
400 | 1
410 | 0

LW_EXCLUSIVE [ms]
value ------------- Distribution ------------- count
< 0 | 0
0 |@@ 1565
10 | 0
20 | 0
30 | 0
40 | 0
50 | 0
60 | 0
70 | 16
80 |@ 894
90 |@@@@@@@ 4108
100 |@@@@@@@@@@@@@ 8090
110 |@@@@@@@@ 4863
120 |@ 595
130 | 64
140 | 14
150 | 0
160 | 4
170 | 93
180 | 262
190 |@ 549
200 |@ 854
210 |@@ 976
220 |@ 770
230 | 268
240 | 40
250 | 2
260 | 9
270 | 29
280 | 46
290 | 94
300 | 134
310 | 169
320 | 164
330 | 146
340 | 94
350 | 13
360 | 2
370 | 8
380 | 15
390 | 17
400 | 20
410 | 37
420 | 27
430 | 32
440 | 29
450 | 8
460 | 4
470 | 2
480 | 1
490 | 3
>= 500 | 48

LW_SHARED [us]
value ------------- Distribution ------------- count
0 | 0
10 | 39
20 |@@@@@@@@@@@@ 31038
30 |@@@@@@@@@@@@@@@@@@@@@@@@@ 66010
40 |@ 2409
50 | 103
60 | 145
70 | 85
80 | 68
90 | 57
100 | 40
110 | 33
120 | 37
130 | 24
140 | 15
150 | 35
160 | 26
170 | 35
180 | 24
190 | 16
200 | 12
210 | 13
220 | 13
230 | 8
240 | 9
250 | 3
260 | 4
270 | 3
280 | 2
290 | 4
300 | 3
310 | 3
320 | 3
330 | 2
340 | 0
350 | 1
360 | 2
370 | 0
380 | 0
390 | 2
400 | 1
410 | 2
420 | 0
430 | 0
440 | 1
450 | 0
460 | 5
470 | 1
480 | 1
490 | 4
500 | 6
510 | 1
520 | 1
530 | 0
540 | 1
550 | 1
560 | 2
570 | 0
580 | 0
590 | 0
600 | 0
610 | 1
620 | 0
630 | 0
640 | 0
650 | 0
660 | 0
670 | 1
680 | 0
690 | 2
700 | 1
710 | 1
720 | 3
730 | 0
740 | 1
750 | 0
760 | 0
770 | 0
780 | 0
790 | 0
800 | 0
810 | 0
820 | 0
830 | 0
840 | 0
850 | 0
860 | 1
870 | 0
880 | 0
890 | 0
900 | 0
910 | 0
920 | 1
930 | 0
940 | 0
950 | 2
960 | 0
970 | 0
980 | 1
990 | 0
>= 1000 |@@@ 6843

LW_EXCLUSIVE [us]
value ------------- Distribution ------------- count
10 | 0
20 |@ 357
30 |@@ 1166
40 | 35
50 | 1
60 | 1
70 | 1
80 | 1
90 | 0
100 | 0
110 | 0
120 | 0
130 | 1
140 | 0
150 | 0
160 | 0
170 | 0
180 | 0
190 | 0
200 | 0
210 | 0
220 | 0
230 | 0
240 | 0
250 | 0
260 | 0
270 | 0
280 | 0
290 | 0
300 | 0
310 | 0
320 | 0
330 | 0
340 | 0
350 | 0
360 | 0
370 | 0
380 | 0
390 | 0
400 | 0
410 | 0
420 | 0
430 | 0
440 | 0
450 | 0
460 | 0
470 | 0
480 | 0
490 | 0
500 | 0
510 | 0
520 | 0
530 | 0
540 | 0
550 | 0
560 | 0
570 | 0
580 | 0
590 | 0
600 | 0
610 | 0
620 | 0
630 | 0
640 | 0
650 | 0
660 | 0
670 | 0
680 | 0
690 | 0
700 | 0
710 | 0
720 | 0
730 | 0
740 | 0
750 | 0
760 | 0
770 | 0
780 | 0
790 | 0
800 | 0
810 | 0
820 | 0
830 | 0
840 | 0
850 | 0
860 | 0
870 | 0
880 | 0
890 | 0
900 | 0
910 | 0
920 | 0
930 | 0
940 | 0
950 | 0
960 | 0
970 | 0
980 | 0
990 | 0
>= 1000 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 23615

--------------------------------------------------------------

#!/usr/sbin/dtrace -s

/*
* Show ProcArrayLock wait time
*
* $Id$
*/

BEGIN
{
self->ts = 0;
}

tick-10sec
{
exit(0);
}

/*
* Accumulate average time spent from start of LWLockAcquire()
* until return from LWLockAcquire().
*/
postgresql$1:::lwlock-acquire-begin
/ arg0 == 4 /
{
self->ts = timestamp;
self->mode = arg1;
}

postgresql$1:::lwlock-acquire-end
/ self->ts /
{
@acqcount[self->mode==0?"Total LW_EXCLUSIVE":"Total LW_SHARED"]
= count();

/* whole frequency distribution 0-500ms */
@wholedist[self->mode==0?"LW_EXCLUSIVE [ms]":"LW_SHARED [ms]"]
= lquantize((timestamp - self->ts)/1000000, 0, 500, 10);

/* the lower part of the distribution in detail 0-1000us */
@lowdist[self->mode==0?"LW_EXCLUSIVE [us]":"LW_SHARED [us]"]
= lquantize((timestamp - self->ts)/1000, 0, 1000, 10);

self->ts = 0;
}

postgresql$1:::transaction-start
{
@acqcount["Total Transaction Starts"] = count();
}

On Wed, 2008-02-06 at 18:44 +0100, Staale Smedseng wrote:
> > What is the frequency distribution of lock wait time on ProcArrayLock?
>
> See below for wait time distributions for ProcArrayLock (both shared and
> exclusive). The time measured is from entry into LWLockAcquire() until
> return. I've recorded the same data in two different resolutions (ms,
> and us for the lower part of the distribution). The DTrace script is at
> the bottom.
>
> These results are for 1000 TPC-C like clients, and measured over the
> 1000 PostgreSQL processes over a period of 10 seconds.

Thanks! Interesting resonance pattern.

--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com

> I'm not sure 32-bit and 64-bit cases are going to be directly
> comparable. We
> could have a problem with cache line aliasing on only one or the
> other for
> example.

Agreed, this is likely comparing apples and oranges. I'll see if I can
get a one-to-one comparison done (these were the numbers I had close
by when writing the email).

> But that is a pretty striking difference. Does the 8.3 run complete
> more
> transactions in that time?

I'll make sure to include those numbers as well. :)

Staale

Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> "Staale Smedseng" <Staale(dot)Smedseng(at)Sun(dot)COM> writes:
>> Also, an interesting observation is that the hot locks seem to have
>> changed from v8.2 to v8.3, making the ProcArrayLock more contended. See
>> the following outputs:
>>
>> PostgreSQL 8.2 (32-bit):
>> ...
>> PostgreSQL 8.3 (64-bit):
>> ...

> I'm not sure 32-bit and 64-bit cases are going to be directly comparable. We
> could have a problem with cache line aliasing on only one or the other for
> example.

Yeah, I find these numbers highly dubious. AFAIR we didn't do anything
that would have reduced CLogControlLock contention, and we definitely
did work to reduce ProcArrayLock contention, so the claimed results seem
directly opposite to expectation. I am wondering if the waits are being
attributed to the right locks --- I remember such an error in a previous
set of dtrace results, and some of the other details such as claiming
shared lock delays but no exclusive lock delays for FirstLockMgrLock
seem less than credible as well.

regards, tom lane

On Wed, 2008-02-06 at 13:55 -0500, Tom Lane wrote:
> Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> > "Staale Smedseng" <Staale(dot)Smedseng(at)Sun(dot)COM> writes:
> >> Also, an interesting observation is that the hot locks seem to have
> >> changed from v8.2 to v8.3, making the ProcArrayLock more contended. See
> >> the following outputs:
> >>
> >> PostgreSQL 8.2 (32-bit):
> >> ...
> >> PostgreSQL 8.3 (64-bit):
> >> ...
>
> > I'm not sure 32-bit and 64-bit cases are going to be directly comparable. We
> > could have a problem with cache line aliasing on only one or the other for
> > example.
>
> Yeah, I find these numbers highly dubious. AFAIR we didn't do anything
> that would have reduced CLogControlLock contention, and we definitely
> did work to reduce ProcArrayLock contention, so the claimed results seem
> directly opposite to expectation. I am wondering if the waits are being
> attributed to the right locks --- I remember such an error in a previous
> set of dtrace results, and some of the other details such as claiming
> shared lock delays but no exclusive lock delays for FirstLockMgrLock
> seem less than credible as well.

There were only 2 lock delays for FirstLockMgrLock in SHARED mode, so it
seems believable that there were 0 lock delays in EXCLUSIVE mode.

I assumed that Staale is running with clog buffers tweaked? Can you
confirm Staale?

--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com

Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> There were only 2 lock delays for FirstLockMgrLock in SHARED mode, so it
> seems believable that there were 0 lock delays in EXCLUSIVE mode.

Not really, considering the extremely limited use of LW_SHARED in lock.c
(GetLockConflicts is used only by CREATE INDEX CONCURRENTLY, and
GetLockStatusData only by the pg_locks view). For the type of benchmark
that I gather this is, there should be *zero* LW_SHARED acquisitions at
all. And even if there are some, they could only be blocking against
the (undoubtedly much more frequent) LW_EXCLUSIVE acquisitions; it's not
very credible that there is zero contention among the LW_EXCLUSIVE locks
yet a few shared acquirers manage to get burnt.

regards, tom lane

On Wed, 2008-02-06 at 14:42 -0500, Tom Lane wrote:
> Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> > There were only 2 lock delays for FirstLockMgrLock in SHARED mode, so it
> > seems believable that there were 0 lock delays in EXCLUSIVE mode.
>
> Not really, considering the extremely limited use of LW_SHARED in lock.c
> (GetLockConflicts is used only by CREATE INDEX CONCURRENTLY, and
> GetLockStatusData only by the pg_locks view). For the type of benchmark
> that I gather this is, there should be *zero* LW_SHARED acquisitions at
> all. And even if there are some, they could only be blocking against
> the (undoubtedly much more frequent) LW_EXCLUSIVE acquisitions; it's not
> very credible that there is zero contention among the LW_EXCLUSIVE locks
> yet a few shared acquirers manage to get burnt.

...but the total wait time on those lock waits was 24 microseconds. I
hardly call that burnt.

--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com

Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> On Wed, 2008-02-06 at 14:42 -0500, Tom Lane wrote:
>> Not really, considering the extremely limited use of LW_SHARED in lock.c
>> (GetLockConflicts is used only by CREATE INDEX CONCURRENTLY, and
>> GetLockStatusData only by the pg_locks view). For the type of benchmark
>> that I gather this is, there should be *zero* LW_SHARED acquisitions at
>> all. And even if there are some, they could only be blocking against
>> the (undoubtedly much more frequent) LW_EXCLUSIVE acquisitions; it's not
>> very credible that there is zero contention among the LW_EXCLUSIVE locks
>> yet a few shared acquirers manage to get burnt.

> ...but the total wait time on those lock waits was 24 microseconds. I
> hardly call that burnt.

What you are failing to grasp is that the data is simply not credible
(unless perhaps Staale fesses up that his benchmark includes a whole lot
of pg_locks monitoring, in which case I'd want to see it redone without
anyway).

regards, tom lane

On Wed, 2008-02-06 at 19:55, Tom Lane wrote:
> I am wondering if the waits are being
> attributed to the right locks --- I remember such an error in a previous
> set of dtrace results, and some of the other details such as claiming
> shared lock delays but no exclusive lock delays for FirstLockMgrLock
> seem less than credible as well.

Good catch. We've checked the DTrace scripts against the respective
versions of lwlock.h, and the FirstLockMgrLock is off (this is actually
the results for FirstBufMappingLock).

However, this is the last lock in the enum that we trace, the other
lower-numbered lock enums were correctly mapped. (In particular the
ProcArrayLock which we've been puzzled by.)

And also, there was no clog buffer twaking or pg_lock monitoring done
when this benchmark was run, AFAIK.

We'll redo the benchmarks and post new scripts and results.

Staale

Staale Smedseng <Staale(dot)Smedseng(at)Sun(dot)COM> writes:
> Good catch. We've checked the DTrace scripts against the respective
> versions of lwlock.h, and the FirstLockMgrLock is off (this is actually
> the results for FirstBufMappingLock).

> However, this is the last lock in the enum that we trace, the other
> lower-numbered lock enums were correctly mapped. (In particular the
> ProcArrayLock which we've been puzzled by.)

Hmm, do you mean that you're just neglecting to collect any stats about
all the dynamically assigned locks? That doesn't seem like it's going
to offer a very good view of what's going on. I think the most useful
stats would include aggregate wait data for all the lockmgr locks,
all the bufmapping locks, all the buffer content locks, and all the
buffer I/O locks. We'd like to think that contention for those would
be low because of the partitioning ... but the point of measuring is
to find out, not just hope.

regards, tom lane

On Thu, 2008-02-07 at 16:29 +0100, Staale Smedseng wrote:
> On Wed, 2008-02-06 at 19:55, Tom Lane wrote:
> > I am wondering if the waits are being
> > attributed to the right locks --- I remember such an error in a previous
> > set of dtrace results, and some of the other details such as claiming
> > shared lock delays but no exclusive lock delays for FirstLockMgrLock
> > seem less than credible as well.
>
> Good catch. We've checked the DTrace scripts against the respective
> versions of lwlock.h, and the FirstLockMgrLock is off (this is actually
> the results for FirstBufMappingLock).

I just realised you are using a lookup to get the text for the name of
the lock. You used the same lookup table for both releases?

--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com

I dont think my earlier message got through..

We use separate lookup tables for 825 and 83 based on the respective
lwlock.h for that version.

-Jignesh

Simon Riggs wrote:
> On Thu, 2008-02-07 at 16:29 +0100, Staale Smedseng wrote:
>
>> On Wed, 2008-02-06 at 19:55, Tom Lane wrote:
>>
>>> I am wondering if the waits are being
>>> attributed to the right locks --- I remember such an error in a previous
>>> set of dtrace results, and some of the other details such as claiming
>>> shared lock delays but no exclusive lock delays for FirstLockMgrLock
>>> seem less than credible as well.
>>>
>> Good catch. We've checked the DTrace scripts against the respective
>> versions of lwlock.h, and the FirstLockMgrLock is off (this is actually
>> the results for FirstBufMappingLock).
>>
>
> I just realised you are using a lookup to get the text for the name of
> the lock. You used the same lookup table for both releases?
>
>

On Thu, 2008-02-07 at 18:12, Simon Riggs wrote:

> I just realised you are using a lookup to get the text for the name of
> the lock. You used the same lookup table for both releases?

Oh, it wasn't quite that bad. :-) The two DTrace scripts had been
revised to correspond with the two different declarations of LWLockId in
lwlock.h (for 8.2.5 and 8.3 respectively). But somehow the value for the
last lock in our lookup table corresponding to

FirstLockMgrLock = FirstBufMappingLock + NUM_BUFFER_PARTITIONS,

in the enum got turned into n+1 instead of n+NUM_BUFFER_PARTITIONS. The
other locks should have been output correctly, however.

But as Tom pointed out, the dynamic locks were not in the equation. So
now we're measuring all lock waits instead of assuming. :-)

Staale

Last try for the script/results (truncating less significant portions of
output which are too big)

Staale Smedseng wrote:
>
> her locks should have been output correctly, however.
>
> But as Tom pointed out, the dynamic locks were not in the equation. So
> now we're measuring all lock waits instead of assuming. :-)
>
> Staale
>
>
>

Here are some recent runs on 4-quadcore xeons using PostgreSQL 8.3.0
with 10-second runs

Hope it gets through this time

-Jignesh

for about 500users :
Lock Id Mode Count
64 Shared 754
53 Exclusive 846
43 Exclusive 1379
11 Shared 2038
3 Exclusive 2376
47 Exclusive 2900
4 Shared 3194
42 Exclusive 3389
50 Exclusive 3689
41 Exclusive 5280
11 Exclusive 6560
48 Exclusive 10837
7 Exclusive 20599
44 Exclusive 29193
46 Exclusive 30299
4 Exclusive 66303

Lock Id Mode Combined Time (ns)
43 Exclusive 3440644758
47 Exclusive 6392480847
50 Exclusive 7199215298
42 Exclusive 7865126413
11 Exclusive 8503940437
4 Shared 9142071412
61 Shared 10613733376
41 Exclusive 14380504651
48 Exclusive 16631996177
7 Exclusive 17485716438
46 Exclusive 61584135053
44 Exclusive 68889040292
4 Exclusive 177458486936

For about 700 Users:

Lock Id Mode Count
3 Exclusive 2873
4 Shared 3436
47 Exclusive 3555
11 Shared 4098
50 Exclusive 4392
42 Exclusive 4573
11 Exclusive 6209
41 Exclusive 7552
48 Exclusive 12335
7 Exclusive 22469
44 Exclusive 36987
46 Exclusive 38467
4 Exclusive 81453

Lock Id Mode Combined Time (ns)
7 Exclusive 12033761450
43 Exclusive 12217953092
50 Exclusive 15454797539
47 Exclusive 15684068953
42 Exclusive 19365161311
4 Shared 27038955376
48 Exclusive 32990318986
41 Exclusive 48350689916
46 Exclusive 193727946721
44 Exclusive 212025745038
4 Exclusive 713263386624

At 1000 users

Lock Id Mode Count
59 Shared 10475
58 Shared 11062
57 Shared 13726
7 Exclusive 18548
44 Exclusive 29714
46 Exclusive 33886
4 Exclusive 74773
11 Exclusive 79596

Lock Id Mode Combined Time (ns)
43 Exclusive 9067359926
47 Exclusive 12259067669
50 Exclusive 13239372833
42 Exclusive 16534292830
7 Exclusive 23505490039
48 Exclusive 24991948402
41 Exclusive 33874749560
11 Shared 43963854482
4 Shared 64098606143
63 Shared 130988696365
64 Shared 137865936811
61 Shared 140978086498
59 Shared 161661023016
62 Shared 163857754020
58 Shared 167946299990
44 Exclusive 220719773416
57 Shared 245170386594
46 Exclusive 326272389569
11 Exclusive 1312009781095
4 Exclusive 2403450038504

4 is ProcArray..
ClogControlLock is not completely out but I dont see it consistently
like ProcArrayLock. It would show up in few cases if I continue taking
different snapshots of the output (maybe when Checkpoint kicks in or
something , I dont know..)

Lock Id Mode Count
3 Exclusive 160
7 Exclusive 396
11 Shared 828
44 Exclusive 991
46 Exclusive 1093
4 Exclusive 1550
62 Shared 20332
57 Shared 21244
64 Shared 21492
63 Shared 21791
61 Shared 22132
58 Shared 23747
60 Shared 26230
11 Exclusive 158168

Lock Id Mode Combined Time (ns)
64864 Shared 3743818402
10830 Shared 3835685469
89688 Shared 4164484448
84354 Shared 4268747714
870 Shared 4376161411
17078 Shared 4712788740
11 Shared 17718901870
62 Shared 331016914134
57 Shared 340929880655
64 Shared 355793689545
63 Shared 362464073193
61 Shared 362561852786
58 Shared 388597307969
60 Shared 430378269549
11 Exclusive 2641836280591

Now the script

# cat 83_all_wait.d
#!/usr/sbin/dtrace -qs

/*
* Usage : 83_all_wait.d
* Description: Show number and total time of LWLock waits by lock id and
mode
* Author : Jignesh Shah
*/

dtrace:::BEGIN
{
lckmode[0] = "Exclusive";
lckmode[1] = "Shared";
/* Only for reference not used... Refer to src/include/storage/lwlock.h */
lckid[0] = "BufFreelistLock";
lckid[1] = "ShmemIndexLock";
lckid[2] = "OidGenLock";
lckid[3] = "XidGenLock";
lckid[4] = "ProcArrayLock";
lckid[5] = "SInvalLock";
lckid[6] = "FreeSpaceLock";
lckid[7] = "WALInsertLock";
lckid[8] = "WALWriteLock";
lckid[9] = "ControlFileLock";
lckid[10] = "CheckpointLock";
lckid[11] = "CLogControlLock";
lckid[12] = "SubtransControlLock";
lckid[13] = "MultiXactGenLock";
lckid[14] = "MultiXactOffsetControlLock";
lckid[15] = "MultiXactMemberControlLock";
lckid[16] = "RelCacheInitLock";
lckid[17] = "BgWriterCommLock";
lckid[18] = "TwoPhaseStateLock";
lckid[19] = "TablespaceCreateLock";
lckid[20] = "BtreeVacuumLock";
lckid[21] = "AddinShmemInitLock";
lckid[22] = "AutovacuumLock";
lckid[23] = "AutovacuumScheduleLock";
lckid[24] = "SyncScanLock";
lckid[25] = "FirstBufMappingLock";
lckid[26] = "FirstLockMgrLock";

}

postgresql*:::lwlock-startwait
{
self->ts[arg0]=timestamp;
@count[arg0, lckmode[arg1]] = count();
}

postgresql*:::lwlock-endwait
/self->ts[arg0]/
{
@time[arg0 ,lckmode[arg1]] = sum (timestamp - self->ts[arg0]);
self->ts[arg0]=0;
}

dtrace:::END {
printf("\n%20s %15s %15s\n", "Lock Id", "Mode", "Count");
printa("%20d %15s %(at)15d\n",@count);

printf("\n%20s %15s %20s\n", "Lock Id","Mode", "Combined Time (ns)");
printa("%20d %15s %(at)20d\n",@time);
}

tick-10sec
{
exit(0);
}

"Jignesh K. Shah" <J(dot)K(dot)Shah(at)Sun(dot)COM> writes:

> for about 500users :
> For about 700 Users:
> At 1000 users

This is a tangent but are these actual Postgres processes? What's the logic
behind trying to run a 1,000 processes on a box with 16 cpus? They're all just
going to be queuing up for i/o requests or sitting runnable waiting for a
timeslice.

Was this with your patch to raise the size of the clog lru?

What is MaxBackends actually set to for the runs. Is it the same even when
you're actually running with fewer backends?

--
Gregory Stark
EnterpriseDB http://www.enterprisedb.com
Get trained by Bruce Momjian - ask me about EnterpriseDB's PostgreSQL training!

Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> This is a tangent but are these actual Postgres processes? What's the logic
> behind trying to run a 1,000 processes on a box with 16 cpus?

We should certainly be careful about trying to eliminate contention in
this scenario at the cost of making things slower in more normal cases,
but it seems interesting to stress the system just to see what happens.

> Was this with your patch to raise the size of the clog lru?

That's an important question.

> What is MaxBackends actually set to for the runs.

That I think is not. I'm fairly sure there are no performance-relevant
paths in which cost is driven by MaxBackends rather than the actual
current number of live backends. Certainly nothing in or around the
ProcArray would act that way.

regards, tom lane

Tom Lane wrote:
> Gregory Stark <stark(at)enterprisedb(dot)com> writes:
>
>> This is a tangent but are these actual Postgres processes? What's the logic
>> behind trying to run a 1,000 processes on a box with 16 cpus?
>>
>
> We should certainly be careful about trying to eliminate contention in
> this scenario at the cost of making things slower in more normal cases,
> but it seems interesting to stress the system just to see what happens.
>
>
>> Was this with your patch to raise the size of the clog lru?
>>
>
> That's an important question.
>
>
>> What is MaxBackends actually set to for the runs.
>>
>
> That I think is not. I'm fairly sure there are no performance-relevant
> paths in which cost is driven by MaxBackends rather than the actual
> current number of live backends. Certainly nothing in or around the
> ProcArray would act that way.
>
>
> regards, tom lane
>

I guess I was not clear.. It was PostgreSQL 8.3.0 (with no source code
change)
I had compiled it 64-bit with DTRACE enabled.
max-backend was set to 1500 But I dont think that causes any thing to
work slow. But yes the connections are "pre-opened" in the sense when
500 users are actively doing work there are about 1006 postgresql
processes running.

Yes I think I am taking the database to the extreme. But generally there
is some THINK time of 50ms involved so there are time slices available
for other users. Yes Commercial DB can also do pretty well on such
systems so its not unrealistic to expect that PostgreSQL cannot perform
here.

The old idea of stress testing it is to prove that it can go beyond
these 16cores infact our target is about 64-cores soon.

Regards,
Jignesh