Re: Speed up Clog Access by increasing CLOG buffers

On 09/14/2016 05:29 PM, Robert Haas wrote:
> On Wed, Sep 14, 2016 at 12:55 AM, Dilip Kumar <dilipbalaut(at)gmail(dot)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