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Re: buffer assertion tripping under repeat pgbench load

From: Greg Smith <greg(at)2ndQuadrant(dot)com>
To: Greg Stark <stark(at)mit(dot)edu>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Andres Freund <andres(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: buffer assertion tripping under repeat pgbench load
Date: 2012-12-26 23:47:59
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On 12/26/12 5:40 PM, Greg Stark wrote:
> Also, do you have the buffer id of the broken buffer? I wonder if it's
> not just any buffer but always the same same buffer even if it's a
> different block in that buffer.

I just added something looking for that.

Before I got to that I found another crash:

2012-12-26 18:01:42 EST [973]: WARNING:  refcount of base/16384/65553 
blockNum=22140, flags=0x1a7 is 1073741824 should be 0, globally: 0
2012-12-26 18:01:42 EST [973]: WARNING:  buffers with non-zero refcount is 1
-bash-4.1$ export PGPORT=5433
-bash-4.1$ psql -d pgbench -c "select relname,relkind,relfilenode from 
pg_class where relfilenode=65553"
         relname        | relkind | relfilenode
  pgbench_accounts_pkey | i       |       65553

So back to an index again.

> (Or maybe your compiler is laying out these objects
> in a different way from most people's compilers and we're overwriting
> past the end of some other object routinely but yours is the only
> place where it's being laid out preceding a critical data structure)

I doubt there is anything special about this compiler, given that it's 
the standard RedHat 6 build stack cloned via Scientific Linux 6.0.

The two things I expect I'm doing differently than most tests are:

-Using 2GB for shared_buffers
-Running a write heavy test that goes for many hours

It would be nice if this were just something like a memory issue on this 
system.  That I'm getting the same very odd value every time--this 
refcount of 1073741824--makes it seem less random than I expect from bad 
memory.  Once I get a few more crash samples (with buffer ids) I'll shut 
the system down for a pass of memtest86+.

Regardless, I've copied over the same source code and test configuration 
to a similar system here.  If I can reproduce this on a second system, 
I'll push all the details out to the list, hopeful that other people 
might see it too.

Greg Smith   2ndQuadrant US    greg(at)2ndQuadrant(dot)com   Baltimore, MD
PostgreSQL Training, Services, and 24x7 Support

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