Re: [PATCH 8/8] vm: Add an tuning knob for vm.max_writeback_mb

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2009-09-09 at 16:23 +0200, Jan Kara wrote:
>   Well, what I imagined we could do is:
> Have a per-bdi variable 'pages_written' - that would reflect the amount of
> pages written to the bdi since boot (OK, we'd have to handle overflows but
> that's doable).
> 
> There will be a per-bdi variable 'pages_waited'. When a thread should sleep
> in balance_dirty_pages() because we are over limits, it kicks writeback thread
> and does:
>   to_wait =  max(pages_waited, pages_written) + sync_dirty_pages() (or
> whatever number we decide)
>   pages_waited = to_wait
>   sleep until pages_written reaches to_wait or we drop below dirty limits.
> 
> That will make sure each thread will sleep until writeback threads have done
> their duty for the writing thread.
> 
> If we make sure sleeping threads are properly ordered on the wait queue,
> we could always wakeup just the first one and thus avoid the herding
> effect. When we drop below dirty limits, we would just wakeup the whole
> waitqueue.
> 
> Does this sound reasonable?

That seems to go wrong when there's multiple tasks waiting on the same
bdi, you'd count each page for 1/n its weight.

Suppose pages_written = 1024, and 4 tasks block and compute their to
wait as pages_written + 256 = 1280, then we'd release all 4 of them
after 256 pages are written, instead of 4*256, which would be
pages_written = 2048.