Tom Lane wrote:
> Andrew Sullivan <andrew@libertyrms.info> writes:
> > On Sun, Nov 02, 2003 at 01:00:35PM -0500, Tom Lane wrote:
> >> real traction we'd have to go back to the "take over most of RAM for
> >> shared buffers" approach, which we already know to have a bunch of
> >> severe disadvantages.
>
> > I know there are severe disadvantages in the current implementation,
> > but are there in-principle severe disadvantages?
>
> Yes. For one, since we cannot change the size of shared memory
> on-the-fly (at least not portably), there is no opportunity to trade off
> memory usage dynamically between processes and disk buffers. For
> another, on many systems shared memory is subject to being swapped out.
> Swapping out dirty buffers is a performance killer, because they must be
> swapped back in again before they can be written to where they should
> have gone. The only way to avoid this is to keep the number of shared
> buffers small enough that they all remain fairly "hot" (recently used)
> and so the kernel won't be tempted to swap out any part of the region.
Agreed, we can't resize shared memory, but I don't think most OS's swap
out shared memory, and even if they do, they usually have a kernel
configuration parameter to lock it into kernel memory. All the old
unixes locked the shared memory into kernel address space and in fact
this is why many of them required a kernel recompile to increase shared
memory. I hope the ones that have pagable shared memory have a way to
prevent it --- at least FreeBSD does, not sure about Linux.
Now, the disadvantages of large kernel cache, small PostgreSQL buffer
cache is that data has to be transfered to/from the kernel buffers, and
second, we can't control the kernel's cache replacement strategy, and
will probably not be able to in the near future, while we do control our
own buffer cache replacement strategy.
Looking at the advantages/disadvantages, a large shared buffer cache
looks pretty good to me.
-- Bruce Momjian | http://candle.pha.pa.us pgman@candle.pha.pa.us | (610)
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