On (03/15/16 09:46), Minchan Kim wrote: [..] > > yes, > > > > we do less work this way - scan and compact only one class, instead > > of locking and compacting all of them; which sounds reasonable. > > Hmm,, It consumes more memory(i.e., sizeof(work_struct) + sizeof(void *) > + sizeof(bool) * NR_CLASS) as well as kicking many work up to NR_CLASS. yes, it does. not really happy with it either. > I didn't test your patch but I guess I can make worst case scenario. > > * make every class fragmented under 40% > * On the 40% boundary, repeated alloc/free of every class so every free > can schedule work if it was not scheduled. > * Although class fragment is too high, it's not a problem if the class > consumes small amount of memory. hm, in this scenario both solutions are less than perfect. we jump X times over 40% margin, we have X*NR_CLASS compaction scans in the end. the difference is that we queue less works, yes, but we don't have to use workqueue in the first place; compaction can be done asynchronously by a pool's dedicated kthread. so we will just wake_up() the process. > I guess it can make degradation if I try to test on zsmalloc > microbenchmark. > > As well, although I don't know workqueue internal well, thesedays, > I saw a few of mails related to workqueue(maybe, vmstat) and it had > some trouble if system memory pressure is heavy IIRC. yes, you are right. wq provides WQ_MEM_RECLAIM bit for this case -- a special kthread that it will wake up to process works. > My approach is as follows, for exmaple. > > Let's make a global ratio. Let's say it's 4M. ok. should it depend on pool size? min(20% of pool_size, XXMB)? > If zs_free(or something) realizes current fragment is over 4M, > kick compacion backgroud job. yes, zs_free() is the only place that introduces fragmentation. > The job scans from highest to lower class and compact zspages > in each size_class until it meets high watermark(e.g, 4M + 4M /2 = > 6M fragment ratio). ok. > And in the middle of background compaction, if we find it's too > many scan(e.g., 256 zspages or somethings), just bail out the > job for the latency and reschedule it for next time. At the next > time, we can continue from the last size class. ok. I'd probably prefer more simple rules here: -- bail out because it has compacted XXMB so the fragmentation ratio is *expected* to be below the watermark -- nothing to scan anymore compaction is executed concurrently with zs_free()/zs_malloc() calls, it's harder to control/guarantee some global state. overall, no real objections. this approach can work, I think. need to test it. > I know your concern is unncessary scan but I'm not sure it can > affect performance although we try to evaluate performance with > microbenchmark. It just loops and check with zs_can_compact > for 255 size class. -ss -- 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>