On Thu, Feb 05 2015, Tejun Heo wrote: > Hello, Greg. > > On Wed, Feb 04, 2015 at 03:51:01PM -0800, Greg Thelen wrote: >> I think the linux-next low (and the TBD min) limits also have the >> problem for more than just the root memcg. I'm thinking of a 2M file >> shared between C and D below. The file will be charged to common parent >> B. >> >> A >> +-B (usage=2M lim=3M min=2M) >> +-C (usage=0 lim=2M min=1M shared_usage=2M) >> +-D (usage=0 lim=2M min=1M shared_usage=2M) >> \-E (usage=0 lim=2M min=0) >> >> The problem arises if A/B/E allocates more than 1M of private >> reclaimable file data. This pushes A/B into reclaim which will reclaim >> both the shared file from A/B and private file from A/B/E. In contrast, >> the current per-page memcg would've protected the shared file in either >> C or D leaving A/B reclaim to only attack A/B/E. >> >> Pinning the shared file to either C or D, using TBD policy such as mount >> option, would solve this for tightly shared files. But for wide fanout >> file (libc) the admin would need to assign a global bucket and this >> would be a pain to size due to various job requirements. > > Shouldn't we be able to handle it the same way as I proposed for > handling sharing? The above would look like > > A > +-B (usage=2M lim=3M min=2M hosted_usage=2M) > +-C (usage=0 lim=2M min=1M shared_usage=2M) > +-D (usage=0 lim=2M min=1M shared_usage=2M) > \-E (usage=0 lim=2M min=0) > > Now, we don't wanna use B's min verbatim on the hosted inodes shared > by children but we're unconditionally charging the shared amount to > all sharing children, which means that we're eating into the min > settings of all participating children, so, we should be able to use > sum of all sharing children's min-covered amount as the inode's min, > which of course is to be contained inside the min of the parent. > > Above, we're charging 2M to C and D, each of which has 1M min which is > being consumed by the shared charge (the shared part won't get > reclaimed from the internal pressure of children, so we're really > taking that part away from it). Summing them up, the shared inode > would have 2M protection which is honored as long as B as a whole is > under its 3M limit. This is similar to creating a dedicated child for > each shared resource for low limits. The downside is that we end up > guarding the shared inodes more than non-shared ones, but, after all, > we're charging it to everybody who's using it. > > Would something like this work? Maybe, but I want to understand more about how pressure works in the child. As C (or D) allocates non shared memory does it perform reclaim to ensure that its (C.usage + C.shared_usage < C.lim). Given C's shared_usage is linked into B.LRU it wouldn't be naturally reclaimable by C. Are you thinking that charge failures on cgroups with non zero shared_usage would, as needed, induce reclaim of parent's hosted_usage? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>