Re: Spread checkpoint sync

On Tue, Jan 11, 2011 at 5:27 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Tue, Nov 30, 2010 at 3:29 PM, Greg Smith <greg@2ndquadrant.com> wrote:
>> One of the ideas Simon and I had been considering at one point was adding
>> some better de-duplication logic to the fsync absorb code, which I'm
>> reminded by the pattern here might be helpful independently of other
>> improvements.
>
> Hopefully I'm not stepping on any toes here, but I thought this was an
> awfully good idea and had a chance to take a look at how hard it would
> be today while en route from point A to point B.  The answer turned
> out to be "not very", so PFA a patch that seems to work.  I tested it
> by attaching gdb to the background writer while running pgbench, and
> it eliminate the backend fsyncs without even breaking a sweat.

I had been concerned about how long the lock would be held, and I was
pondering ways to do only partial deduplication to reduce the time.

But since you already wrote a patch to do the whole thing, I figured
I'd time it.

I arranged to test an instrumented version of your patch under large
shared_buffers of 4GB, conditions that would maximize the opportunity
for it to take a long time.  Running your compaction to go from 524288
to a handful (14 to 29, depending on run) took between 36 and 39
milliseconds.

For comparison, doing just the memcpy part of AbsorbFsyncRequest on
a full queue took from 24 to 27 milliseconds.

They are close enough to each other that I am no longer interested in
partial deduplication.  But both are long enough that I wonder if
having a hash table in shared memory that is kept unique automatically
at each update might not be worthwhile.

Cheers,

Jeff