On Fri, Jan 18, 2019 at 12:04:17PM +0100, Paolo Valente wrote: > > > > Il giorno 18 gen 2019, alle ore 11:31, Andrea Righi <righi.andrea@xxxxxxxxx> ha scritto: > > > > This is a redesign of my old cgroup-io-throttle controller: > > https://lwn.net/Articles/330531/ > > > > I'm resuming this old patch to point out a problem that I think is still > > not solved completely. > > > > = Problem = > > > > The io.max controller works really well at limiting synchronous I/O > > (READs), but a lot of I/O requests are initiated outside the context of > > the process that is ultimately responsible for its creation (e.g., > > WRITEs). > > > > Throttling at the block layer in some cases is too late and we may end > > up slowing down processes that are not responsible for the I/O that > > is being processed at that level. > > > > = Proposed solution = > > > > The main idea of this controller is to split I/O measurement and I/O > > throttling: I/O is measured at the block layer for READS, at page cache > > (dirty pages) for WRITEs, and processes are limited while they're > > generating I/O at the VFS level, based on the measured I/O. > > > > Hi Andrea, > what the about the case where two processes are dirtying the same > pages? Which will be charged? > > Thanks, > Paolo Hi Paolo, in this case only the first one will be charged for the I/O activity (the one that changes a page from clean to dirty). This is probably not totally fair in some cases, but I think it's a good compromise, at the end rewriting the same page over and over while it's already dirty doesn't actually generate I/O activity, until the page is flushed back to disk. Obviously I'm open to other better ideas and suggestions. Thanks! -Andrea
