On Tue, 2011-02-01 at 16:38 +0100, Andrea Arcangeli wrote: > On Mon, Jan 31, 2011 at 04:33:57PM -0800, Dave Hansen wrote: > > I'm working on some more reports that transparent huge pages and > > KSM do not play nicely together. Basically, whenever THP's are > > present along with KSM, there is a lot of attrition over time, > > and we do not see much overall progress keeping THP's around: > > > > http://sr71.net/~dave/ibm/038_System_Anonymous_Pages.png > > > > (That's Karl Rister's graph, thanks Karl!) > > Well if the pages_sharing/pages_shared count goes up, this is a > feature not a bug.... You need to print that too in the chart to show > this is not ok Here are the KSM sharing bits for the same run: http://sr71.net/~dave/ibm/009_KSM_Pages.png It bounces around a little bit on the ends, but it's fairly static during the test, even when there's a good downward slope on the THP's. Hot of the presses, Karl also managed to do a run last night with the khugepaged scanning rates turned all the way up: http://sr71.net/~dave/ibm/038_System_Anonymous_Pages-scan-always.png The THP's there are a lot more stable. I'd read that as saying that the scanning probably just isn't keeping up with whatever is breaking the pages up. > KSM will slowdown performance also during copy-on-writes when > pages_sharing goes up, not only because of creating non-linearity > inside 2m chunks (which makes mandatory to use ptes and not hugepmd, > it's not an inefficiency of some sort that can be optimized away > unfortunately). We sure could change KSM to merge 2M pages instead of > 4k pages, but then the memory-density would decrease of several order > of magnitudes making the KSM scan almost useless (ok, with guest > heavily using THP that may change, but all pagecache is still 4k... so > for now it'd be next to useless). Yup, unless we do something special, the odds of sharing those 2MB suckers are near zero. > I would prefer to close the issues that you just previously reported, > sometime with mmap_sem and issues like that, before adding more > features though but I don't want to defer things either so it's up to > you. I'm happy to hold on to them for another release. I'm actually going to go look at the freezes I saw now that I have these out in the wild. I'll probably stick them in a git tree and keep them up to date. Are there any other THP issues you're chasing at the moment? -- Dave -- 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/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
