On Mon, Oct 04, 2021 at 10:25:12AM -0700, Shakeel Butt wrote: > > > To evaluate the impact of this patch, an 8 GiB tmpfs file is created on > > > a system with swap-on-zram and the file was pushed to swap through > > > memory.force_empty interface. On reading the whole file, the memcg stat > > > flush in the refault code path is triggered. With this patch, we > > > observed 38% reduction in the read time of 8 GiB file. > > > > The patch looks fine to me but that's a lot of reduction in read time. Can > > you elaborate a bit on why this makes such a huge difference? Who's hitting > > on that lock so hard? > > It was actually due to machine size. I ran a single threaded workload > without any interference on a 112 cpus machine. So, most of the time > the flush was acquiring and releasing the per-cpu rstat lock for empty > trees. Sorry for being so slow but can you point to the exact call path which gets slowed down so significantly? I'm mostly wondering whether we need some sort of time-batched flushes because even with lock avoidance the flush path really isn't great when called frequently. We can mitigate it further if necessary - e.g. by adding an "updated" bitmap so that the flusher doesn't have to go around touching the cachelines for all the cpus. Thanks. -- tejun
