* Avi Kivity <avi@xxxxxxxxxx> [2010-06-15 12:44:31]: > On 06/15/2010 10:49 AM, Balbir Singh wrote: > > > >>All we need is to select the right page to drop. > >> > >Do we need to drop to the granularity of the page to drop? I think > >figuring out the class of pages and making sure that we don't write > >our own reclaim logic, but work with what we have to identify the > >class of pages is a good start. > > Well, the class of pages are 'pages that are duplicated on the > host'. Unmapped page cache pages are 'pages that might be > duplicated on the host'. IMO, that's not close enough. > Agreed, but what happens in reality with the code is that it drops not-so-frequently-used cache (still reusing the reclaim mechanism), but prioritizing cached memory. > >>How can the host tell if there is duplication? It may know it has > >>some pagecache, but it has no idea whether or to what extent guest > >>pagecache duplicates host pagecache. > >> > >Well it is possible in host user space, I for example use memory > >cgroup and through the stats I have a good idea of how much is duplicated. > >I am ofcourse making an assumption with my setup of the cached mode, > >that the data in the guest page cache and page cache in the cgroup > >will be duplicated to a large extent. I did some trivial experiments > >like drop the data from the guest and look at the cost of bringing it > >in and dropping the data from both guest and host and look at the > >cost. I could see a difference. > > > >Unfortunately, I did not save the data, so I'll need to redo the > >experiment. > > I'm sure we can detect it experimentally, but how do we do it > programatically at run time (without dropping all the pages). > Situations change, and I don't think we can infer from a few > experiments that we'll have a similar amount of sharing. The cost > of an incorrect decision is too high IMO (not that I think the > kernel always chooses the right pages now, but I'd like to avoid > regressions from the unvirtualized state). > > btw, when running with a disk controller that has a very large > cache, we might also see duplication between "guest" and host. So, > if this is a good idea, it shouldn't be enabled just for > virtualization, but for any situation where we have a sizeable cache > behind us. > It depends, once the disk controller has the cache and the pages in the guest are not-so-frequently-used we can drop them. Please remember we still use the LRU to identify these pages. > >>It doesn't, really. The host only has aggregate information about > >>itself, and no information about the guest. > >> > >>Dropping duplicate pages would be good if we could identify them. > >>Even then, it's better to drop the page from the host, not the > >>guest, unless we know the same page is cached by multiple guests. > >> > >On the exact pages to drop, please see my comments above on the class > >of pages to drop. > > Well, we disagree about that. There is some value in dropping > duplicated pages (not always), but that's not what the patch does. > It drops unmapped pagecache pages, which may or may not be > duplicated. > > >There are reasons for wanting to get the host to cache the data > > There are also reasons to get the guest to cache the data - it's > more efficient to access it in the guest. > > >Unless the guest is using cache = none, the data will still hit the > >host page cache > >The host can do a better job of optimizing the writeouts > > True, especially for non-raw storage. But even there we have to > fsync all the time to keep the metadata right. > > >>But why would the guest voluntarily drop the cache? If there is no > >>memory pressure, dropping caches increases cpu overhead and latency > >>even if the data is still cached on the host. > >> > >So, there are basically two approaches > > > >1. First patch, proactive - enabled by a boot option > >2. When ballooned, we try to (please NOTE try to) reclaim cached pages > >first. Failing which, we go after regular pages in the alloc_page() > >call in the balloon driver. > > Doesn't that mean you may evict a RU mapped page ahead of an LRU > unmapped page, just in the hope that it is double-cached? > > Maybe we need the guest and host to talk to each other about which > pages to keep. > Yeah.. I guess that falls into the domain of CMM. > >>>2. Drop the cache on either a special balloon option, again the host > >>>knows it caches that very same information, so it prefers to free that > >>>up first. > >>Dropping in response to pressure is good. I'm just not convinced > >>the patch helps in selecting the correct page to drop. > >> > >That is why I've presented data on the experiments I've run and > >provided more arguments to backup the approach. > > I'm still unconvinced, sorry. > The reason for making this optional is to let the administrators decide how they want to use the memory in the system. In some situations it might be a big no-no to waste memory, in some cases it might be acceptable. -- 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
