"Stephen C. Tweedie" <sct@redhat.com> writes:
> Hi,
>
> On Mon, Sep 25, 2000 at 09:32:42PM +0200, Andrea Arcangeli wrote:
>
> > Having shrink_mmap that browse the mapped page cache is useless
> > as having shrink_mmap browsing kernel memory and anonymous pages
> > as it does in 2.2.x as far I can tell. It's an algorithm
> > complexity problem and it will waste lots of CPU.
>
> It's a compromise between CPU cost and Getting It Right. Ignoring the
> mmap is not a good solution either.
>
> > Now think this simple real life example. A 2G RAM machine running
> > an executable image of 1.5G, 300M in shm and 200M in cache.
Hey that's ridiculous: 1.5G executable image and 300M shm? Take it
vice-versa and you are approaching real life.
> OK, and here's another simple real life example. A 2GB RAM machine
> running something like Oracle with a hundred client processes all
> shm-mapping the same shared memory segment.
That sound much more realistic.
> Oh, and you're also doing lots of file IO. How on earth do you decide
> what to swap and what to page out in this sort of scenario, where
> basically the whole of memory is data cache, some of which is mapped
> and some of which is not?
>
> If you don't separate out the propagation of referenced bits from the
> actual page aging, then every time you pass over the whole VM working
> set, you're likely to find a handful of live references to some of the
> shared memory, and a hundred or so references that haven't done
> anything since last time. Anything that only ages per-pte, not
> per-page, is simply going to die horribly under such load, and any
> imbalance between pure filesystem cache and VM pressure will be
> magnified to the point where one dominates.
Yes and that's why I stress most of the patch levels with my ipctst
program on a highmem machine. It's simulating a load like this: A lot
of processes attached to shm segments and trashing them. There were
very few kernels which really worked with that load without totally
breaking or killing processes _way_ too early.
> Hence my observation that it's really easy to find special cases where
> certain optimisations make a ton of sense, but you often lose balance
> in the process.
O.K. My test case is such a special case, but it is related to real
live transactional load on a highend server.
Greetings
Christoph
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