Re: [RFC] [PATCH] Avoid livelock for fsync

[Nick Piggin]

On Mon, Oct 26, 2009 at 07:13:14PM +0100, Jan Kara wrote:
>   Hi,
> 
>   on my way back from Kernel Summit, I've coded the attached patch which
> implements livelock avoidance for write_cache_pages. We tag patches that
> should be written in the beginning of write_cache_pages and then write
> only tagged pages (see the patch for details). The patch is based on Nick's
> idea.
>   The next thing I've aimed at with this patch is a simplification of
> current writeback code. Basically, with this patch I think we can just rip
> out all the range_cyclic and nr_to_write (or other "fairness logic"). The
> rationalle is following:
>   What we want to achieve with fairness logic is that when a page is
> dirtied, it gets written to disk within some reasonable time (like 30s or
> so). We track dirty time on per-inode basis only because keeping it
> per-page is simply too expensive. So in this setting fairness between
> inodes really does not make any sence - why should be a page in a file
> penalized and written later only because there are lots of other dirty
> pages in the file? It is enough to make sure that we don't write one file
> indefinitely when there are new dirty pages continuously created - and my
> patch achieves that.
>   So with my patch we can make write_cache_pages always write from
> range_start (or 0) to range_end (or EOF) and write all tagged pages. Also
> after changing balance_dirty_pages() so that a throttled process does not
> directly submit the IO (Fengguang has the patches for this), we can
> completely remove the nr_to_write logic because nothing really uses it
> anymore. Thus also the requeue_io logic should go away etc...
>   Fengguang, do you have the series somewhere publicly available? You had
> there a plenty of changes and quite some of them are not needed when the
> above is done. So could you maybe separate out the balance_dirty_pages
> change and I'd base my patch and further simplifications on top of that?
> Thanks.

Like I said (and as we concluded when I last posted my tagging patch),
I think this idea should work fine, but there is perhaps a little bit of
overhead/complexity so provided that we can get some numbers or show a
real improvement in behaviour or code simplifications then I think we
could justify the patch.

I would be interested to know how it goes.

Thanks,
Nick
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