On Fri 15-04-16 10:20:33, Colum Paget wrote: > Hi all, > > I suspect that many people will have reported this, but I thought I'd drop > you > a line just in case everyone figures someone else has reported it. It's > possible we're just doing something wrong and so encountering this problem, > but I can't find anyone saying they've found a solution, and the problem > doesn't seem to be present in 3.x kernels, which makes us think it could be a > bug. > > We are seeing a problem in 4.4.5 and 4.4.6 32-bit 'hugemem' kernels running > on > machines with > 4GB ram.
I would generally discourage you from using much more than 4G on 32b system. Lowmem mem pressure is a real problem which is inherent to the highmem kernels. > The problem results in disk performance dropping > from 120 MB/s to 1MB/s or even less. 3.18.x 32-bit kernels do not seem to > exhibit this behaviour, or at least we can't make it happen reliably. We've > tried 3.14.65 and 3.14.65 and they don't exhibit the same degree of problem. I would expect this is due to dirty memory throttling. Highmem is not considered dirtyable normally (see global_dirtyable_memory) and so all the writers will get throttled earlier. Basically any change to how much memory can be dirtied in in the lowmem will change the balance for you. > We've not yet been able to test 64 bit kernels, it will be a while before we > can. We've been able to reproduce the problem on multiple machines with > different hardware configs, and with different kernel configs as regards > SMP , NUMA support and transparent hugepages. > > This problem can be reproduced thusly: Have you tried echo 1 > /proc/sys/vm/highmem_is_dirtyable Please note that this might help but it is a double edge sword because it might cause pre mature OOM killers in certain loads. 32b is simply not that great with a lot of memory. HTH -- Michal Hocko SUSE Labs
