Re: [2.4] heavy-load under swap space shortage

Hugh Dickins
2004-05-26T19:06:58Z

On Wed, 26 May 2004, Marcelo Tosatti wrote:

> Andrea, Hugh, Jun'ichi,
>
> I think we can merge this patch.

I guess so. I'm unenthusiastic since I've never worked out whether
it's _right_, or just an ad hoc hack that happens to work around
more fundamental issues, quite successfully in some workloads.

Andrea seems to have devised it to reduce pagemap_lru_lock
contention on bigiron, yet here it's solving a different problem.
Which may be a sign that it's a great patch, or a sign that we
(I!) don't understand what goes on here well enough.

Please don't count me as against it: I just don't know.

(My involvement was earlier when Jun'ichi reported page_table_lock
contention there. We were working together on an entirely different
kind of patch addressing that issue, when Andrea suggested he try this
vm_anon_lru patch. As I understand it, that solved Jun'ichi's particular
problem much more satisfactorily than our own dabblings; but I rather
dropped out at that point.)

> Its very safe - default behaviour unchanged.

Yes, but please update the comments to reflect that, they imply
vm_anon_lru 0 by default, presumably how it was in Andrea's tree.

The tunability, of course, does unfairly make it look more like a
hack than it is; but if we're uncertain, yes, a tunable hack is
much better than a wrong decision now.

Hugh