Paul Tuckfield wrote:
> Biggest problem I've had in the past w. linux in general is that it
> seems to make poor VM choices under heavy filesystem IO. I don't really
> get exactly where it's going wrong , but I've had numerous experiences
> on older systems where bursty IO would seem to cause paging on the box
> (pageout of pieces of the oracle SGA shared memory) which is a
> performance disaseter. It seems to happen even when the shared memory
> was sized reasonably below the size of physical ram, presumably because
> linux is too aggressive in allocating filesystem cache (?) anyway, it
> seems to make decisions based on desire for zippy workstation
> performance and gets burned on thruput on database servers. I'm
> guessing this may be an issue for you , when doing heavy IO. Thing is,
> it'll show like you're IO bound kindof because you're thrashing.
This is not surprising. There has always been an issue with dynamic
buffer cache systems contending with memory used by processes. It takes
a long time to get the balance right, and still there might be cases
where it gets things wrong. Isn't there a Linux option to lock shared
memory in to RAM? If so, we should document this in our manuals, but
right now, there is no mention of it.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 359-1001
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