Re: Page aging broken in 2.6

[William Lee Irwin III <wli@holomorphy.com>]

On Sun, Dec 28, 2003 at 12:07:58AM +0100, Roger Luethi wrote:
> It can matter. Evicting a page that is infrequently referenced by many
> processes increases the chance that all runnable processes block waiting
> for that same page later. The likelihood of that happening grows under
> memory pressure, when "infrequently" may actually be "quite often" and
> when disk I/O is congested (resulting in higher disk access times).
> You won't have the same effect when evicting a page that is referenced
> by one process only, no matter how frequently.

Part of this is unrealistic; paging I/O being congested must be due to
paging itself causing seeks without additional I/O load. Reading a
single page once and then faulting that one page back into numerous
process address spaces is only one I/O request, and so cannot seek in
and of itself. So in this scenario, a convoy of processes on a single
page is plausible; aggravated paging I/O seekiness is not. Did you have
in mind some additional I/O load? Or do affected processes actually all
fault before the one I/O completes, and so all block temporarily?

On Sun, Dec 28, 2003 at 12:07:58AM +0100, Roger Luethi wrote:
> Having all processes blocked is indeed one problem of 2.6 under memory
> pressure. I don't know what the cause is, though.

Can you capture sysrq t while a situation like this is in progress?

-- wli