Re: [PATCH]: 4/4 cluster page-out in VM scanner

[Hans Reiser]

How well does this integrate with reiser4.;-)

Hans

Nikita Danilov wrote:

> Implement pageout clustering at the VM level.
>
> With this patch VM scanner calls pageout_cluster() instead of
> ->writepage(). pageout_cluster() tries to find a group of dirty pages around
> target page, called "pivot" page of the cluster. If group of suitable size is
> found, ->writepages() is called for it, otherwise, page_cluster() falls back
> to ->writepage().
>
> This is supposed to help in work-loads with significant page-out of
> file-system pages from tail of the inactive list (for example, heavy dirtying
> through mmap), because file system usually writes multiple pages more
> efficiently. Should also be advantageous for file-systems doing delayed
> allocation, as in this case they will allocate whole extents at once.
>
> Few points:
>
> - swap-cache pages are not clustered (although they can be, but by
>   page->private rather than page->index)
>
> - currently, kswapd clusters all the time, and direct reclaim only when
>   device queue is not congested. Probably direct reclaim shouldn't cluster at
>   all.
>
> - this patch adds new fields to struct writeback_control and expects
>   ->writepages() to interpret them. This is needed, because pageout_cluster()
>   calls ->writepages() with pivot page already locked, so that ->writepages()
>   is allowed to only trylock other pages in the cluster.
>
>   Besides, rather rough plumbing (wbc->pivot_ret field) is added to check
>   whether ->writepages() failed to write pivot page for any reason (in latter
>   case page_cluster() falls back to ->writepage()).
>
>   Only mpage_writepages() was updated to honor these new fields, but
>   all in-tree ->writepages() implementations seem to call
>   mpage_writepages(). (Except reiser4, of course, for which I'll send a
>   (trivial) patch, if necessary).
>
> Numbers that talk:
>
> Averaged number of microseconds it takes to dirty 1GB of
> 16-times-larger-than-RAM ext3 file mmaped in 1GB chunks:
>
> without-patch:   average:    74188417.156250
>               deviation:    10538258.613280
>
>   with-patch:   average:    69449001.583333
>               deviation:    12621756.615280
>
> (Patch is for 2.6.10-rc2)
>
>  

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