Re: filecache/swapcache questions [RFC] [RFT] [PATCH] kanoj-mm12-2.3.8Fix swapoff races

["Stephen C. Tweedie" <sct@redhat.com>]

Hi,

On Mon, 28 Jun 1999 21:29:07 -0400 (EDT), Chuck Lever <cel@monkey.org>
said:

> yes, that's exactly what i did.  what i can't figure out is why do the
> shrink_mmap in both places?  seems like the shrink_mmap in kswapd is
> overkill if it has just been awoken by try_to_free_pages.

It hasn't necessarily.  It may have been woken by networking activity.
If the memory requirements are being driven by interrupts, not
processes, then kswapd is the only chance for shrink_mmap to be called.

> stephen also mentioned "rate controlling" a trashing process, but since
> nothing in swap_out spins or sleeps, how could a process be slowed except
> by a little extra CPU time spent behind the global lock?  that will slow
> everyone else down too, yes?

There are IO queue limits which will eventually stall the process.  The
ll_rw_block itself one rate limiter.  We also have a test in
rw_swap_page_base:

	/* Don't allow too many pending pages in flight.. */
	if (atomic_read(&nr_async_pages) > pager_daemon.swap_cluster)
		wait = 1;

which causes the swapout to become synchronous once we have filled the
swapper queues.

> seems like try_to_free_pages ought to make a clear effort to recognize a
> process that is growing quickly and slow it down by causing it to sleep.

It does. 

--Stephen

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