Re: mosbench revisited

Martijn van Oosterhout <kleptog(at)svana(dot)org> writes:
> On Wed, Aug 03, 2011 at 02:21:25PM -0400, Robert Haas wrote:
>> It would be nice if the Linux guys would fix this problem for us, but
>> I'm not sure whether they will. For those who may be curious, the
>> problem is in generic_file_llseek() in fs/read-write.c. On a platform
>> with 8-byte atomic reads, it seems like it ought to be very possible
>> to read inode->i_size without taking a spinlock.

> Interesting. There's this thread from 2003 suggesting the use of pread
> instead, it was rejected on the argument that lseek is cheap so not a
> problem.

> http://archives.postgresql.org/pgsql-patches/2003-02/msg00197.php

That seems rather unrelated. The point here is our use of lseek to find
out the current file size --- or at least, I would hope they're not
trying to read the inode's file size in a SEEK_CUR call.

The reason "-M prepared" helps is presumably that it eliminates most of
the RelationGetNumberOfBlocks calls the planner does to check current
table size. While we could certainly consider using a cheaper (possibly
more stale) value there, it's a bit astonishing to think that that's the
main cost in a parse/plan/execute cycle. Perhaps there are more hotspot
calls than that one?

regards, tom lane