KAMEZAWA Hiroyuki wrote: > On Thu, 22 May 2008 22:26:55 -0400 > Rik van Riel <riel@xxxxxxxxxx> wrote: > >> Even worse is that a cgroup has NO CONTROL over how much >> of its memory is kept in RAM and how much is swapped out. We used to have a control on the swap cache pages as well, but their implementation needed more thought > Could you explain "NO CONTROL" ? cgroup has LRU.... > 'how mucch memory should be swapped out from memory' is well controlled > in the VM besides LRU logic ? > >> This kind of decision is made on a system-wide basis by >> the kernel, dependent on what other processes in the system >> are doing. There also is no easy way for a cgroup to reduce >> its swap use, unlike with other resources. >> One option is to limit the virtual address space usage of the cgroup to ensure that swap usage of a cgroup will *not* exceed the specified limit. Along with a good swap controller, it should provide good control over the cgroup's memory usage. > >> In what scenario would you use a resource controller that >> rewards a group for reaching its limit? >> >> How can the cgroup swap space controller help sysadmins >> achieve performance or fairness goals on a system? >> > Perforamnce is not the first goal of this swap controller, I think. > This is for resouce isolation/overcommiting. > > 1. Some _crazy_ people considers swap as very-slow-memory resource ;) > I don't think so but I know there are tons of people.... > > 2. Resource Isolation. > When a cgroup has memory limitation, it can create tons of swap. > For example, limit a cgroup's memory to be 128M and malloc 3G bytes. > 2.8Gbytes of swap will be used _easily_. A process can use up all swap. > In that case, other process can't use swap. > > IIRC, a man shown his motivation to controll swap in OLS2007/BOF as following. > == > Consider following system. (and there is no swap controller.) > Memory 4G. Swap 1G. with 2 cgroups A, B. > > state 1) swap is not used. > A....memory limit to be 1G no swap usage memory_usage=0M > B....memory limit to be 1G no swap usage memory_usage=0M > > state 2) Run a big program on A. > A....memory limit to be 1G and try to use 1.7G. uses 700MBytes of swap. > memory_usage=1G swap_usage=700M > B....memory_usage=0M > > state 3) A some of programs ends in 'A' > A....memory_usage=500M swap_usage=700M > B....memory_usage=0M. > > state 4) Run a big program on B. > A...memory_usage=500M swap_usage=700M. > B...memory_usage=1G swap_usage=300M > > Group B can only use 1.3G because of unfair swap use of group A. > But users think why A uses 700M of swap with 500M of free memory.... > > If we don't have limitation to swap, we'll have to innovate a way to move swap > to memory in some reasonable logic. > > Thanks, > -Kame -- Warm Regards, Balbir Singh Linux Technology Center IBM, ISTL _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers
