On 1/13/26 01:10, Andres Freund wrote:
> Hi,
>
> On 2026-01-13 00:58:49 +0100, Tomas Vondra wrote:
>> On 1/10/26 02:42, Andres Freund wrote:
>>> psql -Xq -c 'SELECT pg_buffercache_evict_all();' -c 'SELECT numa_node,
>>> sum(size) FROM pg_shmem_allocations_numa GROUP BY 1;' && perf stat
>>> --per-socket -M memory_bandwidth_read,memory_bandwidth_write -a psql -c
>>> 'SELECT sum(abalance) FROM pgbench_accounts;'
>
>> And then I initialized pgbench with scale that is much larger than
>> shared buffers, but fits into RAM. So cached, but definitely > NB/4. And
>> then I ran
>>
>> select * from pgbench_accounts offset 1000000000;
>>
>> which does a sequential scan with the circular buffer you mention abobe
>
> Did you try it with the query I suggested? One plausible reason why you did
> not see an effect with your query is that with a huge offset you actually
> never deform the tuple, which is an important and rather latency sensitive
> path.
>
I did try with the agg query too, and there's still no difference on
either machine.
I can't do the perf on the Azure VM, because the Ubuntu is image is
borked and does not allow installing the package. But on my xeon I can
do the perf, and that gives me this:
numactl --membind=0 --cpunodebind=0 ~/builds/master-test/bin/pg_ctl
-----------------------------------------------------------------------
S0 1 24,677,226 UNC_M_CAS_COUNT.WR # 79.0 MB/s ... idth_write
S0 1 20,001,829,522 ns duration_time ...
S0 1 972,631,426 UNC_M_CAS_COUNT.RD # 3112.2 MB/s ... idth_read
S0 1 20,001,822,807 ns duration_time ...
S1 1 15,602,233 UNC_M_CAS_COUNT.WR # 49.9 MB/s ... idth_write
S1 1 712,431,146 UNC_M_CAS_COUNT.RD # 2279.6 MB/s ... idth_read
numactl --membind=0 --cpunodebind=1 ~/builds/master-test/bin/pg_ctl
-----------------------------------------------------------------------
S0 1 47,931,019 UNC_M_CAS_COUNT.WR # 153.4 MB/s ... idth_write
S0 1 20,002,933,380 ns duration_time ...
S0 1 1,007,386,994 UNC_M_CAS_COUNT.RD # 3223.2 MB/s ... idth_read
S0 1 20,002,927,341 ns duration_time ...
S1 1 10,310,201 UNC_M_CAS_COUNT.WR # 33.0 MB/s ... idth_write
S1 1 714,826,668 UNC_M_CAS_COUNT.RD # 2287.2 MB/s ... idth_read
so there is a little bit of a difference for some stats, but not much.
FWIW this is from
perf stat --per-socket -M memory_bandwidth_read,memory_bandwidth_write
-a -- sleep 20
while the agg query runs in a loop.
cheers
--
Tomas Vondra
