Re: [PATCH][5/?] count writeback pages in nr_scanned

[Nick Piggin]

Andrea Arcangeli wrote:
> On Wed, Jan 05, 2005 at 09:05:39PM -0800, Andrew Morton wrote:
>
> > Andrea Arcangeli <andrea@xxxxxxx> wrote:
> >
> > > The fix is very simple and it is to call wait_on_page_writeback on one
> > > of the pages under writeback.
> >
> > eek, no.  That was causing waits of five seconds or more.  Fixing this
> > caused the single greatest improvement in page allocator latency in early
> > 2.5.  We're totally at the mercy of the elevator algorithm this way.
> >
> > If we're to improve things in there we want to wait on _any_ eligible page
> > becoming reclaimable, not on a particular page.
>
>
> I told you one way to fix it. I didn't guarantee it was the most
> efficient one.
>
> I sure agree waiting on any page to complete writeback is going to fix
> it too. Exactly because this page was a "random" page anyway.
>
> Still my point is that this is a bug, and I prefer to be slow and safe
> like 2.4, than fast and unreliable like 2.6.
>
> The slight improvement you suggested of waiting on _any_ random
> PG_writeback to go away (instead of one particular one as I did in 2.4)
> is going to fix the write throttling equally too as well as the 2.4
> logic, but without introducing slowdown that 2.4 had.
>
> It's easy to demonstrate: exactly because the page we pick is random
> anyway, we can pick the first random one that has seen PG_writeback
> transitioning from 1 to 0. The guarantee we get is the same in terms of
> safety of the write throttling, but we also guarantee the best possible
> latency this way. And the HZ/x hacks to avoid deadlocks will magically
> go away too.

This is practically what blk_congestion_wait does when the queue
isn't congested though, isn't it?
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