Re: Thoughts about NUM_BUFFER_PARTITIONS - Mailing list pgsql-hackers

From Li Japin
Subject Re: Thoughts about NUM_BUFFER_PARTITIONS
Date
Msg-id 5C838C5E-D89C-495C-A0E2-87FC683462CD@hotmail.com
Whole thread Raw
In response to Re: Thoughts about NUM_BUFFER_PARTITIONS  (Tomas Vondra <tomas.vondra@enterprisedb.com>)
Responses Re: Thoughts about NUM_BUFFER_PARTITIONS
List pgsql-hackers

> On Feb 10, 2024, at 20:15, Tomas Vondra <tomas.vondra@enterprisedb.com> wrote:
> 
> On 2/8/24 14:27, wenhui qiu wrote:
>> Hi Heikki Linnakangas
>>    I think the larger shared buffer  higher the probability of multiple
>> backend processes accessing the same bucket slot BufMappingLock
>> simultaneously, (   InitBufTable(NBuffers + NUM_BUFFER_PARTITIONS); When I
>> have free time, I want to do this test. I have seen some tests, but the
>> result report is in Chinese
>> 
> 
> I think Heikki is right this is unrelated to the amount of RAM. The
> partitions are meant to reduce the number of lock collisions when
> multiple processes try to map a buffer concurrently. But the machines
> got much larger in this regard too - in 2006 the common CPUs had maybe
> 2-4 cores, now it's common to have CPUs with ~100 cores, and systems
> with multiple of them. OTOH the time spent holing the partition lock
> should be pretty low, IIRC we pretty much just pin the buffer before
> releasing it, and the backend should do plenty other expensive stuff. So
> who knows how many backends end up doing the locking at the same time.
> 
> OTOH, with 128 partitions it takes just 14 backends to have 50% chance
> of a conflict, so with enough cores ... But how many partitions would be
> enough? With 1024 partitions it still takes only 38 backends to get 50%
> chance of a collision. Better, but considering we now have hundreds of
> cores, not sure if sufficient.
> 
> (Obviously, we probably want much lower probability of a collision, I
> only used 50% to illustrate the changes).
> 

I find it seems need to change MAX_SIMUL_LWLOCKS if we enlarge the NUM_BUFFER_PARTITIONS,
I didn’t find any comments to describe the relation between MAX_SIMUL_LWLOCKS and
NUM_BUFFER_PARTITIONS, am I missing someghing?

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