On Mon, 14 Jun 2010 00:01:45 +0530 Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> wrote: > * Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> [2010-06-08 21:21:46]: > > > Selectively control Unmapped Page Cache (nospam version) > > > > From: Balbir Singh <balbir@xxxxxxxxxxxxxxxxxx> > > > > This patch implements unmapped page cache control via preferred > > page cache reclaim. The current patch hooks into kswapd and reclaims > > page cache if the user has requested for unmapped page control. > > This is useful in the following scenario > > > > - In a virtualized environment with cache=writethrough, we see > > double caching - (one in the host and one in the guest). As > > we try to scale guests, cache usage across the system grows. > > The goal of this patch is to reclaim page cache when Linux is running > > as a guest and get the host to hold the page cache and manage it. > > There might be temporary duplication, but in the long run, memory > > in the guests would be used for mapped pages. > > - The option is controlled via a boot option and the administrator > > can selectively turn it on, on a need to use basis. > > > > A lot of the code is borrowed from zone_reclaim_mode logic for > > __zone_reclaim(). One might argue that the with ballooning and > > KSM this feature is not very useful, but even with ballooning, > > we need extra logic to balloon multiple VM machines and it is hard > > to figure out the correct amount of memory to balloon. With these > > patches applied, each guest has a sufficient amount of free memory > > available, that can be easily seen and reclaimed by the balloon driver. > > The additional memory in the guest can be reused for additional > > applications or used to start additional guests/balance memory in > > the host. > > > > KSM currently does not de-duplicate host and guest page cache. The goal > > of this patch is to help automatically balance unmapped page cache when > > instructed to do so. > > > > There are some magic numbers in use in the code, UNMAPPED_PAGE_RATIO > > and the number of pages to reclaim when unmapped_page_control argument > > is supplied. These numbers were chosen to avoid aggressiveness in > > reaping page cache ever so frequently, at the same time providing control. > > > > The sysctl for min_unmapped_ratio provides further control from > > within the guest on the amount of unmapped pages to reclaim. > > > > Are there any major objections to this patch? > This kind of patch needs "how it works well" measurement. - How did you measure the effect of the patch ? kernbench is not enough, of course. - Why don't you believe LRU ? And if LRU doesn't work well, should it be fixed by a knob rather than generic approach ? - No side effects ? - Linux vm guys tend to say, "free memory is bad memory". ok, for what free memory created by your patch is used ? IOW, I can't see the benefit. If free memory that your patch created will be used for another page-cache, it will be dropped soon by your patch itself. If your patch just drops "duplicated, but no more necessary for other kvm", I agree your patch may increase available size of page-caches. But you just drops unmapped pages. Hmm. Thanks, -Kame -- 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
