Re: More observations...

[Rik van Riel <riel@conectiva.com.br>]

On Tue, 16 May 2000, Stephen C. Tweedie wrote:

> The concept is quite simple: if you can limit a process's RSS,
> you can limit the amount of memory which is pinned in process
> page tables, and thus subject to expensive swapping.  Note that
> you don't have to get rid of the pages --- you can leave them in
> the page cache/swap cache, where they can be re-faulted rapidly
> if needed, but if the memory is needed for something else then
> shrink_mmap can reclaim the pages rapidly.

There's one problem with this idea. The current implementation
of shrink_mmap() skips over dirty pages, leading to a failing
shrink_mmap(), calls to swap_out() and replacement of the wrong
pages...

> Rick's old memory hog flag is essentially a simple case of an
> RSS limit (the task RSS is limited to what it is currently set
> at).

Not really. The anti-hog code did a number of things:
- swap_out() scans tasks more and more agressively the
  bigger their RSS gets bigger, meaning we "push back
  harder" if a process is very big
- slow down the allocation rate of very big processes
  by having them call try_to_free_pages() if they want
  to allocate something. It doesn't have to steal a page
  from itself, but can steal the page from anywhere.

The effect should be comperable to RSS limits, only simpler ;)

(After all, all RSS limits do is make sure that the VM subsystem
"pushes back harder" against the VM pressure of big processes)

regards,

Rik
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