Re: [RFC, POC] Don't require a NBuffer sized PrivateRefCount array of local buffer pins

On 8/26/14, 6:52 PM, Andres Freund wrote:
> On 2014-03-21 19:22:31 +0100, Andres Freund wrote:
>> >Hi,
>> >
>> >I've been annoyed at the amount of memory used by the backend local
>> >PrivateRefCount array for a couple of reasons:
>> >
>> >a) The performance impact of AtEOXact_Buffers() on Assert() enabled
>> >    builds is really, really annoying.
>> >b) On larger nodes, the L1/2/3 cache impact of randomly accessing
>> >    several megabyte big array at a high frequency is noticeable. I've
>> >    seen the access to that to be the primary (yes, really) source of
>> >    pipeline stalls.
>> >c) On nodes with significant shared_memory the sum of the per-backend
>> >    arrays is a significant amount of memory, that could very well be
>> >    used more beneficially.
>> >
>> >So what I have done in the attached proof of concept is to have a small
>> >(8 currently) array of (buffer, pincount) that's searched linearly when
>> >the refcount of a buffer is needed. When more than 8 buffers are pinned
>> >a hashtable is used to lookup the values.
>> >
>> >That seems to work fairly well. On the few tests I could run on my
>> >laptop - I've done this during a flight - it's a small performance win
>> >in all cases I could test. While saving a fair amount of memory.
> Here's the next version of this patch. The major change is that newly

<snip>

> The memory savings are clearly visible. During a pgbench scale 350, -cj
> 128 readonly run the following awk
> for pid in $(pgrep -U andres postgres); do
>      grep VmData/proc/$pid/status;
> done | \
>      awk 'BEGIN { sum = 0 } {sum += $2;} END { if (NR > 0) print sum/NR; else print 0;print sum;print NR}'
>
> shows:
>
> before:
> AVG: 4626.06
> TOT: 619892
> NR: 134
>
> after:
> AVG: 1610.37
> TOT: 217400
> NR: 135

These results look very encouraging, especially thinking about the cache impact. It occurs to me that it'd also be nice
tohave some stats available on how this is performing; perhaps a dtrace probe for whenever we overflow to the hash
table,and one that shows maximum usage for a statement? (Presumably that's not much extra code or overhead...)
 
-- 
Jim C. Nasby, Data Architect                       jim@nasby.net
512.569.9461 (cell)                         http://jim.nasby.net