Re: Speed up Clog Access by increasing CLOG buffers

On Fri, Sep 23, 2016 at 6:50 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Thu, Sep 22, 2016 at 7:44 PM, Tomas Vondra
> <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote:
>> I don't dare to suggest rejecting the patch, but I don't see how we could
>> commit any of the patches at this point. So perhaps "returned with feedback"
>> and resubmitting in the next CF (along with analysis of improved workloads)
>> would be appropriate.
>
> I think it would be useful to have some kind of theoretical analysis
> of how much time we're spending waiting for various locks. So, for
> example, suppose we one run of these tests with various client counts
> - say, 1, 8, 16, 32, 64, 96, 128, 192, 256 - and we run "select
> wait_event from pg_stat_activity" once per second throughout the test.
> Then we see how many times we get each wait event, including NULL (no
> wait event). Now, from this, we can compute the approximate
> percentage of time we're spending waiting on CLogControlLock and every
> other lock, too, as well as the percentage of time we're not waiting
> for lock. That, it seems to me, would give us a pretty clear idea
> what the maximum benefit we could hope for from reducing contention on
> any given lock might be.
>

As mentioned earlier, such an activity makes sense, however today,
again reading this thread, I noticed that Dilip has already posted
some analysis of lock contention upthread [1]. It is clear that patch
has reduced LWLock contention from ~28% to ~4% (where the major
contributor was TransactionIdSetPageStatus which has reduced from ~53%
to ~3%). Isn't it inline with what you are looking for?

[1] - https://www.postgresql.org/message-id/CAFiTN-u-XEzhd%3DhNGW586fmQwdTy6Qy6_SXe09tNB%3DgBcVzZ_A%40mail.gmail.com

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com