On Mon, Apr 13, 2020 at 09:24:14AM +0800, Huang, Ying wrote: > Andrea Righi <andrea.righi@xxxxxxxxxxxxx> writes: > > > Introduce a new fixed-size swap-in readahead policy that can be selected > > at run-time. > > > > The global swap-in readahead policy takes in account the previous access > > patterns, using a scaling heuristic to determine the optimal readahead > > chunk dynamically. > > > > This works pretty well in most cases, but like any heuristic there are > > specific cases when this approach is not ideal, for example the swapoff > > scenario. > > > > During swapoff we just want to load back into memory all the swapped-out > > pages and for this specific use case a fixed-size readahead is more > > efficient. > > > > This patch introduces a new sysfs interface > > (/sys/kernel/mm/swap/swap_ra_policy) that can be set as following: > > > > - 0: current scaling swap-in readahead policy (default) > > - 1: fixed-size readahead policy (size is determined by > > vm.page-cluster) > > > > The specific use case this patch is addressing is to improve swapoff > > performance when a VM has been hibernated, resumed and all memory needs > > to be forced back to RAM by disabling swap (see the test case below). > > I don't know whether the use case is important or not. Usually the > performance of swapoff doesn't matter. Maybe you can provide some > information on this. In general yes, I agree, swapoff performance is not that important. This particular optimization has been tested a lot in the Ubuntu AWS kernel and it seems to speed up consistently the resume of hibernated EC2 instances (see also [1]). So we were considering to upstream this, if we find a way to make it more suitable for mainline inclusion. [1] https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/Hibernate.html > > Even if it's important, I don't think we really need to add another ABI. > You can just optimize the swapoff performance inside kernel. And we may > not even need to add a new policy, just use cluster readahead and > increase swapin_readahead_hits in swapoff if cluster readahead is used. OK, instead of adding a new ABI I can make this optimization directly in the kernel, so that when swapoff runs it can automatically use the fixed-size cluster readahead and continue using the default readahead for everything else. I'll play a bit with this and will send a new patch. Thanks! -Andrea
