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

[Andrea Arcangeli]

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.
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