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Re: Group commit, revised

From: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
To: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Peter Geoghegan <peter(at)2ndquadrant(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, PG Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Group commit, revised
Date: 2012-01-29 00:48:53
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Fri, Jan 27, 2012 at 5:35 AM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> On 26.01.2012 04:10, Robert Haas wrote:
>> I think you should break this off into a new function,
>> LWLockWaitUntilFree(), rather than treating it as a new LWLockMode.
>> Also, instead of adding lwWaitOnly, I would suggest that we generalize
>> lwWaiting and lwExclusive into a field lwWaitRequest, which can be set
>> to 1 for exclusive, 2 for shared, 3 for wait-for-free, etc.  That way
>> we don't have to add another boolean every time someone invents a new
>> type of wait - not that that should hopefully happen very often.  A
>> side benefit of this is that you can simplify the test in
>> LWLockRelease(): keep releasing waiters until you come to an exclusive
>> waiter, then stop.  This possibly saves one shared memory fetch and
>> subsequent test instruction, which might not be trivial - all of this
>> code is extremely hot.
> Makes sense. Attached is a new version, doing exactly that.

Others are going to test this out on high-end systems.  I wanted to
try it out on the other end of the scale.  I've used a Pentium 4,
with 2GB of RAM and with a single IDE drive running ext4.  ext4 is
amazingly bad on IDE, giving about 25 fsync's per second (and it lies
about fdatasync, but apparently not about fsync)

I ran three modes, head, head with commit_delay, and the group_commit patch

shared_buffers = 600MB

optionally with:

pgbench -i -s40

for clients in 1 5 10 15 20 25 30
pgbench -T 30 -M prepared -c $clients -j $clients

ran 5 times each, taking maximum tps from the repeat runs.

The results are impressive.

clients	head	head_commit_delay	group_commit
1	23.9	23.0	23.9
5	31.0	51.3	59.9
10	35.0	56.5	95.7
15	35.6	64.9	136.4
20	34.3	68.7	159.3
25	36.5	64.1	168.8
30	37.2	83.8	71.5

I haven't inspected that deep fall off at 30 clients for the patch.

By way of reference, if I turn off synchronous commit, I get
tps=1245.8 which is 100% CPU limited.  This sets an theoretical upper
bound on what could be achieved by the best possible group committing

If the group_commit patch goes in, would we then rip out commit_delay
and commit_siblings?



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