On 10/11/2017 02:26 AM, pinker wrote:
> Tomas Vondra-4 wrote
>> I'm probably a bit dumb (after all, it's 1AM over here), but can you
>> explain the CPU chart? I'd understand percentages (say, 75% CPU used)
>> but what do the seconds / fractions mean? E.g. when the system time
>> reaches 5 seconds, what does that mean?
>
> hehe, no you've just spotted a mistake, it suppose to be 50 cores :)
> out of 80 in total
>
Ah, so it should say '50 cores' instead of '5s'? Well, that's busy
system I guess.
>
> Tomas Vondra-4 wrote
>> Have you tried profiling using perf? That usually identifies hot spots
>> pretty quickly - either in PostgreSQL code or in the kernel.
>
> I was always afraid because of overhead, but maybe it's time to start ...
>
I don't follow. If you're not in trouble, a little bit of additional
overhead is not an issue (but you generally don't need profiling at that
moment). If you're already in trouble, then spending a bit of CPU time
on basic CPU profile is certainly worth it.
>
> Tomas Vondra-4 wrote
>> What I meant is that if the system evicts this amount of buffers all the
>> time (i.e. there doesn't seem to be any sudden spike), then it's
>> unlikely to be the cause (or related to it).
>
> I was actually been thinking about scenario where different sessions
> want to at one time read/write from or to many different relfilenodes,
> what could cause page swap between shared buffers and os cache?
Perhaps. If the sessions only do reads, that would not be visible in
buffer_backends I believe (not sure ATM, would have to check source).
But it'd be visible in buffers_alloc and certainly in blks_read.
> we see that context switches on cpu are increasing as well. kernel
> documentation says that using page tables instead of Translation
> Lookaside Buffer (TLB) is very costly and on some blogs have seen
> recomendations that using huge pages (so more addresses can fit in
> TLB) will help here but postgresql, unlike oracle, cannot use it for
> anything else than page buffering (so 16gb) ... so process memory
> still needs to use 4k pages.
>
The context switches are likely due to large number of runnable
processes competing for the CPU.
Also, memory bandwidth is increasingly an issue on big boxes ...
regards
--
Tomas Vondra http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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