Re: 2.5.48-mm1

[Bill Davidsen <davidsen@tmr.com>]

On Tue, 19 Nov 2002, Hugh Dickins wrote:

> I disagree with this one (changing balance_dirty_pages to _ratelimited
> when loop_thread writes to file): it's a step in the right direction,
> but I think you should remove that balance_dirty_pages call completely.
> 
> I'm experimenting with what's needed to prevent deadoralivelock in
> loop over tmpfs under heavy memory pressure (thank you for eliminating
> wait_on_page_bit from shrink_list!).  One element of that is to ignore
> balance_dirty_pages below loop (I hadn't noticed the explicit call,
> offhand I'm unsure whether that's the only possible instance).
> 
> The loop_thread is working towards undirtying memory (completing
> writeback): a loop of blk_congestion_waits is appropriate at the
> upper level where the user task generating dirt needs to be throttled,
> but I don't believe it's appropriate at this level - we wouldn't want
> to throttle the disk, no more should we throttle the loop_thread.

This is purely a performance decision. If you want to avoid bad latency on
reads then you have to throttle writes. The loop_thread will make the
system just as slow as a user application writing the same number of
pages.

If you want io scheduling you will deliberately slow writes to let reads
happen in reasonable time. And vice-versa I imagine, although I don't
think I've seen that case.

-- 
bill davidsen <davidsen@tmr.com>
  CTO, TMR Associates, Inc
Doing interesting things with little computers since 1979.

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