Subject: Re: Speed up Clog Access by increasing CLOG buffers
To: Robert Haas <robertmhaas@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>
References: 
 <CAA4eK1+8=X9mSNeVeHg_NqMsOR-XKsjuqrYzQf=iCsdh3U4EOA@mail.gmail.com>
 <20150903114137.GE27649@awork2.anarazel.de>
 <CAA4eK1LMMGNQ439BUm0LcS3p0sb8S9kc-cUGU_ThNqMwA8_Tug@mail.gmail.com>
 <CAA4eK1+-=18HOrdqtLXqOMwZDbC_15WTyHiFruz7BvVArZPaAw@mail.gmail.com>
 <20160330230914.GH13305@awork2.anarazel.de>
 <CAA4eK1L2KO18G5-ajNitztnmn1G_Ex1N4oUHhF0Xx3QrY57Ufw@mail.gmail.com>
 <20160408153054.hgubabeycowlu3tc@alap3.anarazel.de>
 <CAA4eK1+6URLZYM+xjnvc027+mqQztWc8VDm0udY2X+bdC2HP+A@mail.gmail.com>
 <8bab43dd-bc76-0c41-950e-8101269dbf4b@2ndquadrant.com>
 <CAA4eK1JJOWO6u99gUMMQWELXbci428vRBbJnxg=V04NuZwBCSw@mail.gmail.com>
 <CAFiTN-u3=XUi7z8dTOgxZ98E7gL1tzL=q9Yd=CwWCtTtS6pOZw@mail.gmail.com>
 <CA+TgmoYO1wNi7dRwNxGUupmMPhOkae-pmU38ndC1s6FDQ-USJg@mail.gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Andres Freund
 <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <0733fb9c-c3dc-fe30-1ad1-8c3ddf05c92d@2ndquadrant.com>
Date: Sat, 17 Sep 2016 05:47:31 +0200
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:46.0) Gecko/20100101
 Thunderbird/46.0a2
MIME-Version: 1.0
In-Reply-To: 
 <CA+TgmoYO1wNi7dRwNxGUupmMPhOkae-pmU38ndC1s6FDQ-USJg@mail.gmail.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org

On 09/14/2016 05:29 PM, Robert Haas wrote:
> On Wed, Sep 14, 2016 at 12:55 AM, Dilip Kumar <dilipbalaut@gmail.com> wrote:
>> 2. Results
>> ./pgbench -c $threads -j $threads -T 10 -M prepared postgres -f script.sql
>> scale factor: 300
>> Clients   head(tps)        grouplock(tps)          granular(tps)
>> -------      ---------               ----------                   -------
>> 128        29367                 39326                    37421
>> 180        29777                 37810                    36469
>> 256        28523                 37418                    35882
>>
>>
>> grouplock --> 1) Group mode to reduce CLOGControlLock contention
>> granular  --> 2) Use granular locking model
>>
>> I will test with 3rd approach also, whenever I get time.
>>
>> 3. Summary:
>> 1. I can see on head we are gaining almost ~30 % performance at higher
>> client count (128 and beyond).
>> 2. group lock is ~5% better compared to granular lock.
>
> Sure, but you're testing at *really* high client counts here.  Almost
> nobody is going to benefit from a 5% improvement at 256 clients.  You
> need to test 64 clients and 32 clients and 16 clients and 8 clients
> and see what happens there.  Those cases are a lot more likely than
> these stratospheric client counts.
>

Right. My impression from the discussion so far is that the patches only 
improve performance with very many concurrent clients - but as Robert 
points out, almost no one is running with 256 active clients, unless 
they have 128 cores or so. At least not if they value latency more than 
throughput.

So while it's nice to improve throughput in those cases, it's a bit like 
a tree falling in the forest without anyone around.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


-- 
Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-hackers