Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-15 Thread Simon Riggs
On Fri, 2004-06-11 at 19:25, Tom Lane wrote:
 Simon Riggs [EMAIL PROTECTED] writes:
  Minor patch to correct erroneous warning in cvs tip, believed to be a
  very minor regression.
 
 This patch is wrong; it effectively disables the warning altogether.
 
  When a shutdown was requested within CHECKPOINT_SECONDS of a checkpoint,
  the shutdown code in the bgwriter (which has does checkpointing) would
  issue the erroneous message:
 
 I don't think so ... the shutdown path doesn't go through this code.
 Could you take another look at the cause of what you saw?

Hmmm...perhaps I should submit this as a *low priority* bug then, rather
than just a patch?

As of now, (i.e. even including the new bgwriter shutdown) if you:
1. start postmaster
2. do some work that writes xlog
3. shutdown within some few seconds of startup

you get a WARNING suggesting you increase CHECKPOINT_SEGMENTS, which is
clearly erroneous since no checkpoint has taken place since startup.

Not exactly a common circumstance, I grant you, but it does seem to be a
regression nonetheless. My thinking was that the circumstance was not
limited to this edge case, but actually any shutdown within 30s of a
checkpoint, but the latter is just speculation.

Only reason I spotted it is I've been doing a lot of startup/shutdown
work recently...

Best Regards, Simon Riggs



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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-15 Thread Tom Lane
Simon Riggs [EMAIL PROTECTED] writes:
 As of now, (i.e. even including the new bgwriter shutdown) if you:
 1. start postmaster
 2. do some work that writes xlog
 3. shutdown within some few seconds of startup
 you get a WARNING suggesting you increase CHECKPOINT_SEGMENTS, which is
 clearly erroneous since no checkpoint has taken place since startup.

[ scratches head... ]  Not for me.  Are you using any nondefault
settings in postgresql.conf?

regards, tom lane

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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-15 Thread Simon Riggs
On Tue, 2004-06-15 at 19:33, Tom Lane wrote:
 Simon Riggs [EMAIL PROTECTED] writes:
  As of now, (i.e. even including the new bgwriter shutdown) if you:
  1. start postmaster
  2. do some work that writes xlog
  3. shutdown within some few seconds of startup
  you get a WARNING suggesting you increase CHECKPOINT_SEGMENTS, which is
  clearly erroneous since no checkpoint has taken place since startup.
 
 [ scratches head... ]  Not for me.  Are you using any nondefault
 settings in postgresql.conf?

Hmmm...I did a complete re-download of CVS, then a vanilla
configure/make/make install with .conf straight from can. I wanted
to be absolutely certain I had got it clean.

Shutdown was via CTRL-Cmake a difference?

Damn I hate the wierd ones...if only because of the 90% chance its
something wrong that I've done...

I'll do it again more slowly and report back...

Best Regards, Simon Riggs



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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-15 Thread Tom Lane
Simon Riggs [EMAIL PROTECTED] writes:
 Shutdown was via CTRL-Cmake a difference?

Wouldn't think so.

I can force the message to appear if I do a *manual* CHECKPOINT command
within thirty seconds of startup.  I'm not sure if that should be
considered wrong or not, but in any case it doesn't seem to be what
you're describing.

regards, tom lane

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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-15 Thread Andrew Dunstan
Tom Lane wrote:
Simon Riggs [EMAIL PROTECTED] writes:
 

Shutdown was via CTRL-Cmake a difference?
   

Wouldn't think so.
I can force the message to appear if I do a *manual* CHECKPOINT command
within thirty seconds of startup.  I'm not sure if that should be
considered wrong or not, but in any case it doesn't seem to be what
you're describing.

Just to confirm that Simon is not suffering this uniquely, I saw this 
the other day on Windows, I believe - meant to report it but it got away 
from me.

cheers
andrew
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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-15 Thread Tom Lane
Andrew Dunstan [EMAIL PROTECTED] writes:
 Just to confirm that Simon is not suffering this uniquely, I saw this 
 the other day on Windows, I believe - meant to report it but it got away 
 from me.

Oh, I bet I know what's going on --- are you guys launching the
postmaster in a console window and then typing ^C to shut it down?
Depending on your shell that might result in SIGINT being delivered
not only to the postmaster but to all its children.  Backends, if
any, will take that as a query-cancel while the bgwriter will take
it as a checkpoint request.  Which could easily result in the
too-frequent-checkpoints message.

The only real fix I can see for this is to use some other signal than
SIGINT to transmit checkpoint requests to the bgwriter.  Maybe we could
piggyback all bgwriter requests onto SIGUSR2, comparable to the way that
signals up to the postmaster are now handled.  Not sure how important
it is though.

regards, tom lane

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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-15 Thread Simon Riggs
On Tue, 2004-06-15 at 21:04, Tom Lane wrote:
 Andrew Dunstan [EMAIL PROTECTED] writes:
  Just to confirm that Simon is not suffering this uniquely, I saw this 
  the other day on Windows, I believe - meant to report it but it got away 
  from me.
 
 Oh, I bet I know what's going on --- are you guys launching the
 postmaster in a console window and then typing ^C to shut it down?
 Depending on your shell that might result in SIGINT being delivered
 not only to the postmaster but to all its children.  Backends, if
 any, will take that as a query-cancel while the bgwriter will take
 it as a checkpoint request.  Which could easily result in the
 too-frequent-checkpoints message.
 
 The only real fix I can see for this is to use some other signal than
 SIGINT to transmit checkpoint requests to the bgwriter.  Maybe we could
 piggyback all bgwriter requests onto SIGUSR2, comparable to the way that
 signals up to the postmaster are now handled.  Not sure how important
 it is though.
 

Sounds like the cause to me...that would explain why it has just started
happening...

Its minor then... I would only do that for testing...

Regards, Simon



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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-15 Thread Bruce Momjian
Tom Lane wrote:
 Simon Riggs [EMAIL PROTECTED] writes:
  Shutdown was via CTRL-Cmake a difference?
 
 Wouldn't think so.
 
 I can force the message to appear if I do a *manual* CHECKPOINT command
 within thirty seconds of startup.  I'm not sure if that should be
 considered wrong or not, but in any case it doesn't seem to be what
 you're describing.

It is wrong.  The message should not appear if you issue CHECKPOINT.

I just did CHECKPOINT;CHECKPOINT and got the warning in the logs.  This
needs to be fixed.

-- 
  Bruce Momjian|  http://candle.pha.pa.us
  [EMAIL PROTECTED]   |  (610) 359-1001
  +  If your life is a hard drive, |  13 Roberts Road
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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-15 Thread Tom Lane
Bruce Momjian [EMAIL PROTECTED] writes:
 I just did CHECKPOINT;CHECKPOINT and got the warning in the logs.  This
 needs to be fixed.

See code:

/*
 * Ideally we should only warn if this checkpoint was
 * requested due to running out of segment files, and not
 * if it was manually requested.  However we can't tell the
 * difference with the current signalling mechanism.
 */

I could argue that a client-driven process that issues CHECKPOINT every
few seconds is equally deserving of a warning.  The only thing wrong is
that the HINT is inapplicable ... but that's why it's a HINT and not
part of the main message.

regards, tom lane

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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-15 Thread Bruce Momjian
Tom Lane wrote:
 Bruce Momjian [EMAIL PROTECTED] writes:
  I just did CHECKPOINT;CHECKPOINT and got the warning in the logs.  This
  needs to be fixed.
 
 See code:
 
 /*
  * Ideally we should only warn if this checkpoint was
  * requested due to running out of segment files, and not
  * if it was manually requested.  However we can't tell the
  * difference with the current signalling mechanism.
  */
 
 I could argue that a client-driven process that issues CHECKPOINT every
 few seconds is equally deserving of a warning.  The only thing wrong is
 that the HINT is inapplicable ... but that's why it's a HINT and not
 part of the main message.

Also consider they could have issued a checkpoint right after the system
did one.  Yuck.

When I added the warning I hoped to only have it happen for full logs
and not CHECKPOINT, but I guess I couldn't and someone else realized
that and added that clearer comment, or originally I could do that, but
since it has been moved into the bgwriter, it can't anymore.

-- 
  Bruce Momjian|  http://candle.pha.pa.us
  [EMAIL PROTECTED]   |  (610) 359-1001
  +  If your life is a hard drive, |  13 Roberts Road
  +  Christ can be your backup.|  Newtown Square, Pennsylvania 19073

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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-15 Thread Tom Lane
Bruce Momjian [EMAIL PROTECTED] writes:
 Tom Lane wrote:
 I could argue that a client-driven process that issues CHECKPOINT every
 few seconds is equally deserving of a warning.  The only thing wrong is
 that the HINT is inapplicable ... but that's why it's a HINT and not
 part of the main message.

 Also consider they could have issued a checkpoint right after the system
 did one.  Yuck.

 When I added the warning I hoped to only have it happen for full logs
 and not CHECKPOINT, but I guess I couldn't and someone else realized
 that and added that clearer comment, or originally I could do that, but
 since it has been moved into the bgwriter, it can't anymore.

I believe the original implementation in the postmaster had a somewhat
different set of bugs ;-).  IIRC it did not react to manual checkpoints
but it did confuse WAL checkpoints with timeout-driven checkpoints.
The present bgwriter can distinguish the third but not the first two.

If we were willing to take the time to generalize the
backend-to-bgwriter signaling mechanism then we could distinguish
WAL-driven checkpoints from manually issued checkpoints.  I'm sort of
intending to do that anyway.  The question stands though: why isn't it
appropriate to warn of overly-frequently-issued manual checkpoints?

regards, tom lane

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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-15 Thread Bruce Momjian
Tom Lane wrote:
 Bruce Momjian [EMAIL PROTECTED] writes:
  Tom Lane wrote:
  I could argue that a client-driven process that issues CHECKPOINT every
  few seconds is equally deserving of a warning.  The only thing wrong is
  that the HINT is inapplicable ... but that's why it's a HINT and not
  part of the main message.
 
  Also consider they could have issued a checkpoint right after the system
  did one.  Yuck.
 
  When I added the warning I hoped to only have it happen for full logs
  and not CHECKPOINT, but I guess I couldn't and someone else realized
  that and added that clearer comment, or originally I could do that, but
  since it has been moved into the bgwriter, it can't anymore.
 
 I believe the original implementation in the postmaster had a somewhat
 different set of bugs ;-).  IIRC it did not react to manual checkpoints
 but it did confuse WAL checkpoints with timeout-driven checkpoints.
 The present bgwriter can distinguish the third but not the first two.
 
 If we were willing to take the time to generalize the
 backend-to-bgwriter signaling mechanism then we could distinguish
 WAL-driven checkpoints from manually issued checkpoints.  I'm sort of
 intending to do that anyway.  The question stands though: why isn't it
 appropriate to warn of overly-frequently-issued manual checkpoints?

Imagine pgbench issuing a checkpoint, which it does.  That could trigger
the warning when the parameter really shouldn't be increased.  To me,
the warning is for cases when you are filling up the WAL logs too
quickly and checkpoints are happening too frequently.  If a user is
doing checkpoints, it isn't anything increasing the checkpoint segments
is going to help.

If you can't fix the code to distinguish between manual and wal-full
checkpoints, we can adjust the warning logic to warn when there are 3
checkpoints in a short period, rather than just two.

-- 
  Bruce Momjian|  http://candle.pha.pa.us
  [EMAIL PROTECTED]   |  (610) 359-1001
  +  If your life is a hard drive, |  13 Roberts Road
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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-15 Thread Tom Lane
Bruce Momjian [EMAIL PROTECTED] writes:
 Tom Lane wrote:
 ... The question stands though: why isn't it
 appropriate to warn of overly-frequently-issued manual checkpoints?

 ... the warning is for cases when you are filling up the WAL logs too
 quickly and checkpoints are happening too frequently.  If a user is
 doing checkpoints, it isn't anything increasing the checkpoint segments
 is going to help.

No, I think the warning is for when checkpoints are happening too
frequently, period.  An overly small checkpoint_segments setting
is one possible cause of that, but the performance penalty from
too many checkpoints is just as bad no matter what's causing it.
(Remember that a checkpoint not only forces I/O in itself, but
significantly increases subsequent WAL traffic because of needing
to dump whole page images into WAL.)

How do you feel about improving the signaling mechanism but using
it just to vary the HINT?

LOG: checkpoints are occurring too frequently (nn seconds apart)
HINT: Consider increasing the configuration parameter checkpoint_segments.

LOG: checkpoints are occurring too frequently (nn seconds apart)
HINT: Issuing explicit CHECKPOINTs so often is really expensive.

and so on.

regards, tom lane

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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-15 Thread Bruce Momjian
Tom Lane wrote:
 Bruce Momjian [EMAIL PROTECTED] writes:
  Tom Lane wrote:
  ... The question stands though: why isn't it
  appropriate to warn of overly-frequently-issued manual checkpoints?
 
  ... the warning is for cases when you are filling up the WAL logs too
  quickly and checkpoints are happening too frequently.  If a user is
  doing checkpoints, it isn't anything increasing the checkpoint segments
  is going to help.
 
 No, I think the warning is for when checkpoints are happening too
 frequently, period.  An overly small checkpoint_segments setting
 is one possible cause of that, but the performance penalty from
 too many checkpoints is just as bad no matter what's causing it.
 (Remember that a checkpoint not only forces I/O in itself, but
 significantly increases subsequent WAL traffic because of needing
 to dump whole page images into WAL.)
 
 How do you feel about improving the signaling mechanism but using
 it just to vary the HINT?
 
 LOG: checkpoints are occurring too frequently (nn seconds apart)
 HINT: Consider increasing the configuration parameter checkpoint_segments.
 
 LOG: checkpoints are occurring too frequently (nn seconds apart)
 HINT: Issuing explicit CHECKPOINTs so often is really expensive.

Sure, fine by me.  My only point is that we need something to tell
people they need to increase their checkpoint_segments.  If we add other
warnings, that is fine too.

-- 
  Bruce Momjian|  http://candle.pha.pa.us
  [EMAIL PROTECTED]   |  (610) 359-1001
  +  If your life is a hard drive, |  13 Roberts Road
  +  Christ can be your backup.|  Newtown Square, Pennsylvania 19073

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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-11 Thread Tom Lane
Simon Riggs [EMAIL PROTECTED] writes:
 Minor patch to correct erroneous warning in cvs tip, believed to be a
 very minor regression.

This patch is wrong; it effectively disables the warning altogether.

 When a shutdown was requested within CHECKPOINT_SECONDS of a checkpoint,
 the shutdown code in the bgwriter (which has does checkpointing) would
 issue the erroneous message:

I don't think so ... the shutdown path doesn't go through this code.
Could you take another look at the cause of what you saw?

regards, tom lane

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Re: [PATCHES] Fix for erroneous warning on Shutdown

2004-06-11 Thread Bruce Momjian

Patch rejected, asking for more research.

---

Simon Riggs wrote:
 
 Minor patch to correct erroneous warning in cvs tip, believed to be a
 very minor regression.
 
 When a shutdown was requested within CHECKPOINT_SECONDS of a checkpoint,
 the shutdown code in the bgwriter (which has does checkpointing) would
 issue the erroneous message:
 
 LOG:  checkpoints are occurring too frequently (%d seconds apart)
 HINT:  Consider increasing the configuration parameter
 checkpoint_segments.
 
 Clearly, this should only occur when specific checkpoint requests have
 been made, shutdown checkpoints should not be included in the warning.
 
 Best regards, Simon Riggs

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-- 
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  [EMAIL PROTECTED]   |  (610) 359-1001
  +  If your life is a hard drive, |  13 Roberts Road
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