Re: Linux-2.1.129..

Dr. Werner Fink (werner@suse.de)
Fri, 20 Nov 1998 13:09:48 +0100

On Thu, Nov 19, 1998 at 10:58:30PM +0100, Rik van Riel wrote:
> On Thu, 19 Nov 1998, Dr. Werner Fink wrote:
>
> > Yes on a 512MB system it's a great win ... on a 64 system I see
> > something like a ``swapping weasel'' under high load.
> >
> > It seems that page ageing or something *similar* would be nice
> > for a factor 512/64 >= 2 ... under high load and not enough
> > memory it's maybe better if we could get the processes in turn
> > into work instead of useless swapping (this was a side effect
> > of page ageing due to the implicit slow down).
>
> It was certainly a huge win when page aging was implemented,
> but we mainly felt that because there used to be an obscure
> bug in vmscan.c, causing the kernel to always start scanning
> at the start of the process' address space.
>
> Now that bug is fixed, it might just be better to switch
> to a multi-queue system. A full implementation of that
> will have to wait until 2.3, but we can easily do an
> el-cheapo simulation of it by simply not freeing swap
> cached pages on the first pass of shrink_mmap().

Hmmm ... we need something real for 2.2 ... so,
let's analyse the problem

If the average time slice of the processes is eaten up by
swapping page back *and* if these pages are spapped out
during to the next time slice the system becomes unusable
(freeing swap cached pages on the first pass of shrink_mmap()
does force this behaviour at high stress).

Therefore we need something like a page ageing which does not
mean that the old scheme is required.

Pages which are swapped in need a higher life time in
physical memory. If a page can be shared this life
time could be a bigger one.
If a process counts such pages up to a limit his pages
should not get a higher life for the next few cycles.

This simple scheme should be implementable in a easy way,
shouldn't it? The appropiate places are

ipc/shm.c::shm_swap_in()
mm/page_alloc.c::swap_in()

and the old place of the old age_page():

mm/vmscan.c::try_to_swap_out()

together with some unused variables out of

include/linux/sched.h::task_struct (e.g. dec_flt)
include/linux/sched.h::struct page (e.g. unused :-)

nothing more is needed due to the better swap cache of
2.1.129 in comparision to 2.0.36.

Werner

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