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Re: extending relations more efficiently

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: extending relations more efficiently
Date: 2012-05-01 14:56:49
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On Tue, May 1, 2012 at 10:22 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> Fair enough, but my understanding was that tests showed that the
> extension lock was a bottleneck, so doing extensions in larger chunks
> should reduce the time we spend waiting for a lock and thus improve
> performance. So while your results here show no gain, there is gain to
> be had elsewhere as a result.

Maybe, but I'm skeptical.  There are a few cases - such as the problem
I fixed with SInvalReadLock - where the actual overhead of taking and
releasing the lock is the problem.  ProcArrayLock has some
as-yet-unfixed issues in this area as well.  But most of our locking
bottlenecks are caused by doing too much work while holding the lock,
not by acquiring and releasing it too frequently.  A single lock
manager partition can cope with upwards of 30k acquire/release cycles
per second, which would amount to >240MB/sec of file extension.  It
seems very unlikely that we're hitting our head against that ceiling,
although maybe you'd like to suggest a test case.   Rather, I suspect
that it's just plain taking too long to perform the actual extension.
If we could extend by 8 blocks in only 4 times the time it takes to
extend by 1 block, then obviously there would be a win available, but
the test results suggest that isn't the case.


Maybe the solution here isn't extending in larger chunks, but allowing
several extensions to proceed in parallel.  It seems likely that a big
chunk of the system call time is being used up waiting for write() to
copy data from user space to kernel space, and there's no reason
several backends couldn't do that in parallel.  I think it would be
sufficient to ensure that nobody starts using block N until the
initial writes of all blocks < N have completed.

Robert Haas
The Enterprise PostgreSQL Company

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