The thing that happened to your servers is that drop_caches frees up page frames in Linux that have not been referenced for a long time. Linux starts to use them for something else, and requires z/VM to page it in. To make room for that, VM will page out other parts of the guests.
You're still missing the point. We don't drop the caches because Linux needs memory but because we want to determine what memory to give back to VM. Yes, you also lose the good things from cache, but that's a trade-off to make with performance instrumentation. As I started my first response in this thread, if you don't plan to give the pages back with CMM or CMMA, it is not a good idea to use drop_caches and swappiness might be a better way to help workloads that get in trouble. Rob On 15 August 2013 19:16, Mauro Souza <thoriu...@gmail.com> wrote: > I have read it now. And as I think there's no CMM involved, I still > disagree. > I said I would never ever drop caches because I saw it almost killing a > LPAR. I had a customer that put drop_caches on crontab, and at that time > *every* guest started dropping caches and zVM was crazy doing a lot of > page-ins and page-outs. Nobody understood why the system was unresponsible. > I think is less costly to zVM to handle dozens of page-ins and outs for a > Linux needing 10MB of memory here and there, than to drop 2GB of caches at > the same time. If you drop all the cache at the same time, Linux will have > to make an I/O operation for every file it will read, and zVM will have to > read a lot of data from PAGE that will be just ignored. > No matter if you drop all the cache at the same time, or drop a page when > you need it, zVM will have to bring it back. Dropping all caches is like > paying a $100k loan in one time, instead of splitting it in 1000 times of > $100. > > > Mauro > http://mauro.limeiratem.com - registered Linux User: 294521 > Scripture is both history, and a love letter from God. > > > 2013/8/15 Rob van der Heij <rvdh...@gmail.com> > > > On 15 August 2013 17:34, Mauro Souza <thoriu...@gmail.com> wrote: > > > > > > > You can safely ignore the cache usage on Linux, zVM will realise the > page > > > was not in use and drop it itself. When Linux needs memory, it will > > reclaim > > > cache pages automatically. > > > > > > So we disagree. Maybe we wouldn't if you had read the page that I > linked > > to. > > > > The point that you miss is when Linux needs memory and reclaims a page > > frame from page cache, z/VM will first bring back the old contents that > > Linux did not care about anymore. This is where getting a free page will > > slow you down. Especially since the LRU patterns interfere. The idea is > > that you want to drop the backing page from z/VM either before it is > paged > > out or before z/VM will page it back in. If you have CMMA enabled, then > > having Linux free the page (by drop_caches) will prevent z/VM from paging > > it out. > > > > Rob > > > > ---------------------------------------------------------------------- > > For LINUX-390 subscribe / signoff / archive access instructions, > > send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or > > visit > > http://www.marist.edu/htbin/wlvindex?LINUX-390 > > ---------------------------------------------------------------------- > > For more information on Linux on System z, visit > > http://wiki.linuxvm.org/ > > > > ---------------------------------------------------------------------- > For LINUX-390 subscribe / signoff / archive access instructions, > send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or > visit > http://www.marist.edu/htbin/wlvindex?LINUX-390 > ---------------------------------------------------------------------- > For more information on Linux on System z, visit > http://wiki.linuxvm.org/ > ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to lists...@vm.marist.edu with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390 ---------------------------------------------------------------------- For more information on Linux on System z, visit http://wiki.linuxvm.org/
