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. 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. > > 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 _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers