On Thu, Mar 15, 2012 at 08:01:52PM +0200, Avi Kivity wrote: > On 03/15/2012 07:49 PM, Dan Magenheimer wrote: > > > One of the potential problems with tmem is reduction in performance when > > > the cache hit rate is low, for example when streaming. > > > > > > Can you test this by creating a large file, for example with > > > > > > dd < /dev/urandom > file bs=1M count=100000 > > > > > > and then measuring the time to stream it, using > > > > > > time dd < file > /dev/null > > > > > > with and without the patch? > > > > > > Should be done on a cleancache enabled guest filesystem backed by a > > > virtio disk with cache=none. > > > > > > It would be interesting to compare kvm_stat during the streaming, with > > > and without the patch. > > > > Hi Avi -- > > > > The "WasActive" patch (https://lkml.org/lkml/2012/1/25/300) > > is intended to avoid the streaming situation you are creating here. > > It increases the "quality" of cached pages placed into zcache > > and should probably also be used on the guest-side stubs (and/or maybe > > the host-side zcache... I don't know KVM well enough to determine > > if that would work). > > > > As Dave Hansen pointed out, the WasActive patch is not yet correct > > and, as akpm points out, pageflag bits are scarce on 32-bit systems, > > so it remains to be seen if the WasActive patch can be upstreamed. > > Or maybe there is a different way to achieve the same goal. > > But I wanted to let you know that the streaming issue is understood > > and needs to be resolved for some cleancache backends just as it was > > resolved in the core mm code. > > Nice. This takes care of the tail-end of the streaming (the more > important one - since it always involves a cold copy). What about the > other side? Won't the read code invoke cleancache_get_page() for every > page? (this one is just a null hypercall, so it's cheaper, but still > expensive). That is something we should fix - I think it was mentioned in the frontswap email thread the need for batching and it certainly seems required as those hypercalls aren't that cheap. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
