On 05/22/2013 12:03 PM, Bob Liu wrote: > > On 05/22/2013 02:57 AM, Seth Jennings wrote: >> On Tue, May 21, 2013 at 02:26:07PM +0800, Bob Liu wrote: >>> In my understanding, currenlty zswap have a few problems. >>> 1. The zswap pool size is 20% of total memory that's too random and once it >>> gets full the performance may even worse because everytime pageout() an anon >>> page two disk-io write ops may happend instead of one. >> >> Just to clarify, 20% is a default maximum amount that zswap can occupy. >> >> Also, in the steady over-the-limit state, the average number of writebacks is >> equal to the number of pages coming into zswap. The description above makes it >> sound like there is a reclaim amplification effect (two writebacks per zswap >> store) when, on average, there is none. The 2:1 effect only happens on one or >> two store operations right after the pool becomes full. > > I don't think it only happens on one or two store operations. > > When the system enter a situation or run a workload which have many anon > pages, the zswap pool will be full easily and most of the time. > > But after it's full there are still many anon pages need to be reclaimed > and frontswap_store() will be entered and call zbud_reclaim_page() to > writeout two pages every time. > > The effect to the user will be after the zswap is full, the disk IO is > always twice than disable it. > >> >> This is unclear though, mostly because the pool limit is enforced in >> zswap. A situation exists where there might be an unbuddied zbud page with >> room for the upcoming allocation but, because we are over the pool limit, >> reclaim is done during that store anyway. I'm working on a clean way to fix > > Yes, but always reclaim by writing out two pages. > So after the pool is full, there will be always more disk IO than normal > which can cause performance drop and make the user surprise. > I found my concern is the same as Mel mentioned about nchunks. Setting nchunks=2 is a way to workaround this issue. But what I'm trying to do is dynamically change the pool size hoping to reduce zswap_is_full() happen. And do the writeback in background in a shrinker instead of the direct path pageout() > frontswap_store(). -- Regards, -Bob -- 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>