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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>