On 05/22/2013 10:08 PM, Seth Jennings wrote: > On Wed, May 22, 2013 at 12:03:03PM +0800, 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. > > I think the part missing here is the just because a page is reclaimed on a > particular store because we are over the zswap limit doesn't necessarily mean > that page will be reallocated to the pool on the next zbud_alloc(). The > reclaimed page is only reallocated if there is no unbuddied page in the pool > with enough free space to hold the requested allocation. > > In the case that the reclaimed page is not reallocated to the pool, we will be > under the pool limit on the next zswap store and not do reclaim. > That's true, I see your idea here. But it's probably that there will be no suitable unbuddied pages in the pool. Mel gave a very good and detail example about nchunks in thread: Re: [PATCHv11 2/4] zbud: add to mm/ -- 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>