Re: [PATCH v2] mm/zswap: change zswap to writethrough cache

[Seth Jennings]

On Fri, Nov 22, 2013 at 11:29:16AM -0600, Seth Jennings wrote:
> On Wed, Nov 20, 2013 at 02:49:33PM -0500, Dan Streetman wrote:
> > Currently, zswap is writeback cache; stored pages are not sent
> > to swap disk, and when zswap wants to evict old pages it must
> > first write them back to swap cache/disk manually.  This avoids
> > swap out disk I/O up front, but only moves that disk I/O to
> > the writeback case (for pages that are evicted), and adds the
> > overhead of having to uncompress the evicted pages, and adds the
> > need for an additional free page (to store the uncompressed page)
> > at a time of likely high memory pressure.  Additionally, being
> > writeback adds complexity to zswap by having to perform the
> > writeback on page eviction.
> > 
> > This changes zswap to writethrough cache by enabling
> > frontswap_writethrough() before registering, so that any
> > successful page store will also be written to swap disk.  All the
> > writeback code is removed since it is no longer needed, and the
> > only operation during a page eviction is now to remove the entry
> > from the tree and free it.
> 
> I like it.  It gets rid of a lot of nasty writeback code in zswap.
> 
> I'll have to test before I ack, hopefully by the end of the day.
> 
> Yes, this will increase writes to the swap device over the delayed
> writeback approach.  I think it is a good thing though.  I think it
> makes the difference between zswap and zram, both in operation and in
> application, more apparent. Zram is the better choice for embedded where
> write wear is a concern, and zswap being better if you need more
> flexibility to dynamically manage the compressed pool.

One thing I realized while doing my testing was that making zswap
writethrough also impacts synchronous reclaim.  Zswap, as it is now,
makes the swapcache page clean during swap_writepage() which allows
shrink_page_list() to immediately reclaim it.  Making zswap writethrough
eliminates this advantage and swapcache pages must be scanned again
before they can be reclaimed, as is the case with normal swapping.

Just something I am thinking about.

Seth
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