* KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> [2010-06-14 09:28:19]: > 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. I can run other benchmarks as well, I will do so > - 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 ? I believe in LRU, just that the problem I am trying to solve is of using double the memory for caching the same data (consider kvm running in cache=writethrough or writeback mode, both the hypervisor and the guest OS maintain a page cache of the same data). As the VM's grow the overhead is substantial. In my runs I found upto 60% duplication in some cases. > > - 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. > Free memory is good for cases when you want to do more in the same system. I agree that in a bare metail environment that might be partially true. I don't have a problem with frequently used data being cached, but I am targetting a consolidated environment at the moment. Moreover, the administrator has control via a boot option, so it is non-instrusive in many ways. > 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. > unmapped and unused are the best targets, I plan to add slab cache control later. -- Three Cheers, Balbir -- 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
