On Thu, Aug 19, 2010 at 07:51:06PM +0800, Johannes Weiner wrote: > On Thu, Aug 19, 2010 at 12:07:31AM +0800, Wu Fengguang wrote: > > On Thu, Aug 19, 2010 at 12:06:13AM +0800, Wu Fengguang wrote: > > > On Wed, Aug 18, 2010 at 04:13:08PM +0200, Johannes Weiner wrote: > > > > Hi Matthew, > > > > > > > > On Tue, Aug 17, 2010 at 03:50:01PM -0400, Matthew Wilcox wrote: > > > > > > > > > > No comment on this? Was it just that I posted it during the VM summit? > > > > > > > > I have not forgotten about it. I just have a hard time reproducing > > > > those extreme stalls you observed. > > > > > > > > Running that test on a 2.5GHz machine with 2G of memory gives me > > > > stalls of up to half a second. The patchset I am experimenting with > > > > gets me down to peaks of 70ms, but it needs further work. > > > > > > > > Mapped file pages get two rounds on the LRU list, so once the VM > > > > starts scanning, it has to go through all of them twice and can only > > > > reclaim them on the second encounter. > > > > > > > > At that point, since we scan without making progress, we start waiting > > > > for IO, which is not happening in this case, so we sit there until a > > > > timeout expires. > > > > > > Right, this could lead to some 1s stall. Shaohua and me also noticed > > > this when investigating the responsiveness issues. And we are wondering > > > if it makes sense to do congestion_wait() only when the bdi is really > > > congested? There are no IO underway anyway in this case. > > I am currently trying to get rid of all the congestion_wait() in the VM. > They are used for different purposes, so they need different replacement > mechanisms. > > I saw Shaohua's patch to make congestion_wait() cleverer. But I really > think that congestion is not a good predicate in the first place. Why > would the VM care about IO _congestion_? It needs a bunch of pages to > complete IO, whether the writing device is congested is not really > useful information at this point, I think. > > > > > since I can not reproduce your observations, I don't know if this is > > > > the (sole) source of the problem. Can I send you patches? > > > > > > Sure. > > Cool! congestion_wait() isn't the sole source in my test. with congestion_wait() removed, the max latency is ~50ms. while if I made the mmaped page reclaimed in one round (makes page_check_references return PAGEREF_RECLAIM_CLEAN for mmaped pages) in the test, the max latency is ~150us. Thanks, Shaohua -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>