Re: Question about locking and pg_locks

<div class="moz-cite-prefix">Il 10/09/2016 23:07, Jeff Janes ha scritto:<br /></div><blockquote
cite="mid:CAMkU=1zuN3KeMzb0Wsi6GaHnc=rt-9HycMVB37N1specYwhoMA@mail.gmail.com"type="cite"><div dir="ltr"><div
class="gmail_extra"><divclass="gmail_quote">On Thu, Sep 8, 2016 at 4:30 AM, Moreno Andreo <span dir="ltr"><<a
href="mailto:moreno.andreo@evolu-s.it"moz-do-not-send="true" target="_blank">moreno.andreo@evolu-s.it</a>></span>
wrote:<br/><blockquote class="gmail_quote" style="margin:0px 0px 0px             0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">Hi folks! :-)<br /><br /> This morning I was woken up by a call of a coworker
screaming"Help, our Postgres server is throwing strange errors!"<br /> Not the best way to start your day...<br /><br
/>OK, to the serious part.<br /><br /> "Strange errors" were (in postgresql-9.1-main.log)<br /> WARNING: out of shared
memory<br/> ERROR: out of shared memory<br /> HINT: you may need to increase max_locks_per_transaction<br /><br />
RestartingPostgresql solved the issue (for now), but that's what I'm wondering:<br /> - the greatest part of this locks
areused by rubyrep (that we use to replicate users' databases), no new users since 3 weeks, first time error show up in
almost2 years<br /> - I read this: <a href="https://wiki.postgresql.org/wiki/Lock_Monitoring" moz-do-not-send="true"
rel="noreferrer"target="_blank">https://wiki.postgresql.org/wi<wbr />ki/Lock_Monitoring</a> but still I can't figure
outwhat to do if I need to know if I have to be worried or not :-)<br /> - I have<br /> OS: Ubuntu 12.04 (won't upgrade
becausewe are leaving this server to a new one with Debian Jessie)<br /> PG: 9.1.6 (same as above, in new server ve
have9.5.4)<br /></blockquote><div><br /></div><div>You have a problem now.  Upgrading PG now might help you solve the
problemyou have now.  Why would you choose to work with one hand tied behind your back now, just because you were
alreadyplanning on upgrading later?  Also, moving both the OS and the PG version at the same time is great if
everythinggoes well.  But if everything doesn't go well, you have greatly increased the scope of the problem-solving by
makingtwo changes at once.  If it were me (and my employer gave me the freedom to do my job effectively), I'd rather
spendmy time bringing forward the date on which I upgrade PG, rather than spend that time tracking down problems that
occur,or at least are hard to track down, because I am running an obsolete PG.  Just double max_locks_per_transaction
(with32GB of ram, that should not be a problem) and call it good until after the
upgrade.</div></div></div></div></blockquote>This migration has to be done in steps, some databases have already been
migratedto new server (and they seem OK for now), some are still on the old server.<br /> Since these steps can't be
thatfast and migration will last at least 2 months, I was trying to track down the problem to see if there was some
connectionwith a particular user activity, so if there were chances for it to come back.<br /> I think I'll definitely
setmax_locks_per_transaction to 128 and focus on the migration (You're right about changing OS, but since Ubuntu and
Debianare same family I thought it should't be that risky...).<br /><br /><blockquote
cite="mid:CAMkU=1zuN3KeMzb0Wsi6GaHnc=rt-9HycMVB37N1specYwhoMA@mail.gmail.com"type="cite"><div dir="ltr"><div
class="gmail_extra"><divclass="gmail_quote"><div> <br /><br /></div><blockquote class="gmail_quote" style="margin:0px
0px0px             0.8ex;border-left:1px solid             rgb(204,204,204);padding-left:1ex"> RAM: 32 GB<br />
shared_buffers= 2GB<br /> max_connections=800<br /> max_locks_per_transaction=64 (default value)<br />
max_prepared_transactions= 0<br /><br /> so, I should be able to manage 800*64 = 5120 locks, right?<br /><br /> Now my
pg_lockstable has more than 6200 rows, but if I reorder them by pid I see that one of them has 5800 of them, and it
keepson eating locks.<br /> If I dig more and get pid info, its state is "<IDLE> in transaction"<br
/></blockquote><div><br/></div><div>On PG9.2 or above, you would be able to see the most recently run statement, in
additionto the state.  That could help a lot in figuring out how it doing this lock-fest (of course you can already get
theclient_hostname and the application_name, which could also be a big help).<br /></div><div><br /></div><div>What is
thedistribution of locks by type?</div><div><br /></div><div>select locktype, count(*) from pg_locks group by 1;<br
/></div></div></div></div></blockquote><br/> virtualxid            | 27<br /> transactionid      | 133<br />
relation              | 775<br /><br /> (number of rows in pg_locks is quite small at the moment...)<br /><br
/><blockquotecite="mid:CAMkU=1zuN3KeMzb0Wsi6GaHnc=rt-9HycMVB37N1specYwhoMA@mail.gmail.com" type="cite"><div
dir="ltr"><divclass="gmail_extra"><div class="gmail_quote"><div><br /></div><div> <br /></div><blockquote
class="gmail_quote"style="margin:0px 0px 0px             0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex"><br/> ATM there are no locks that have granted = false.<br /><br /> Now, question
time:<br/> - Is there a number of pg_locks rows to be worried about? At more than 6000 I'm still not facing out of
sharedmemory again<br /></blockquote><div><br /></div><div>I don't think that the exact number of locks that will fit
inshared memory is predictable.  For example, if different processes co-hold sharable locks on the same list of tables,
itseems to take up slightly more memory than if each process was locking a different list tables.  And different lock
typesalso take different amounts of memory.  And memory fragmentation might also cause changes in capacity that are
hardto predict--I can run the same parallel program repeated, and have it sometimes run out of memory and sometimes
not.<br/></div></div></div></div></blockquote> Ok, got it.<br /><blockquote
cite="mid:CAMkU=1zuN3KeMzb0Wsi6GaHnc=rt-9HycMVB37N1specYwhoMA@mail.gmail.com"type="cite"><div dir="ltr"><div
class="gmail_extra"><divclass="gmail_quote"><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px
     0.8ex;border-left:1px solid             rgb(204,204,204);padding-left:1ex"> - Is there a way to release locks of
thatpid without pg_terminate() it?<br /></blockquote><div><br /></div><div>I'm afraid not.  I believe your two options
arepg_terminate_backend or things which are functionally equivalent to that; or to identify who is doing this (see
application_nameand client_hostname) and haranguing them until they stop doing it.</div></div></div></div></blockquote>
OK.<br/><blockquote cite="mid:CAMkU=1zuN3KeMzb0Wsi6GaHnc=rt-9HycMVB37N1specYwhoMA@mail.gmail.com" type="cite"><div
dir="ltr"><divclass="gmail_extra"><div class="gmail_quote"><div><br /></div><div><br /></div><div>Cheers,</div><div><br
/></div><div>Jeff</div></div></div></div></blockquote><p>Thanks,<p>Moreno.-<br/>