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Date: Fri, 13 Mar 2009 14:48:33 -0400
From: "Jignesh K. Shah" <J.K.Shah@Sun.COM>
Subject: Re: Proposal of tunable fix for scalability of 8.4
In-reply-to: <C5DFEA86.342D%scott@richrelevance.com>
To: Scott Carey <scott@richrelevance.com>
Cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Message-id: <49BAAA81.2080408@sun.com>
References: <C5DFEA86.342D%scott@richrelevance.com>
User-Agent: Thunderbird 2.0.0.19 (Macintosh/20081209)

Somebody else asked a question: This is actually a two socket machine 
(128) threads but one socket is disabled by the OS so only 64-threads 
are available... The idea being let me choke one socket first with 100% 
CPU ..
> Forgot some data: with the second test above, CPU: 48% user, 18% sys, 
> 35% idle. CPU increased from 46% used in the first test to 65% used, 
> the corresponding throughput increase was not as large, but that is 
> expected on an 8-threads per core server since memory bandwidth and 
> cache resources at a minimum are shared and only trivial tasks can 
> scale 100%.
>
> Based on the above, I would guess that attaining closer to 100% 
> utilization (its hard to get past 90% with that many cores no matter 
> what), will probablyl give another 10 to 15% improvement at most, to 
> maybe 180000/min throughput.
>
> Its also rather interesting that the 2000 connection case with wait 
> times gets 170000/min throughput and beats the 328 users with 0 delay 
> result above. I suspect the ‘wake all’ version is just faster. I would 
> love to see a ‘wake all shared, leave exclusives at front of queue’ 
> version, since that would not allow lock starvation. 
Considering that there is one link list it is just easier to wake the 
sequential selected few or wake them all up.. If I go through the list 
trying to wake all the shared ones then I essentially need to have 
another link list to collect all the exclusives ...

I will retry the thundering herd of waking all waiters irrespective of 
shared, exclusive and see how that behaves.. I think the biggest benefit 
is when the process is waked up and the process in reality is already on 
the cpu checking the field to see whether last guy who released the lock 
is allowing him to wake up or not.

Still I will try some more experiments.. Definitely reducing time in 
"Waiting" lock waits benefits and making "Acquired" times more efficient 
results in more tpm per user.

I will try another run with plain wake up all and see with the same 
parameters (0 think time) that test behaves..

-Jignesh

-- 
Jignesh Shah           http://blogs.sun.com/jkshah  			
The New Sun Microsystems,Inc   http://sun.com/postgresql