Jens Axboe - 12.12.19, 16:16:31 CET: > On 12/12/19 3:44 AM, Martin Steigerwald wrote: > > Jens Axboe - 11.12.19, 16:29:38 CET: > >> Recently someone asked me how io_uring buffered IO compares to > >> mmaped > >> IO in terms of performance. So I ran some tests with buffered IO, > >> and > >> found the experience to be somewhat painful. The test case is > >> pretty > >> basic, random reads over a dataset that's 10x the size of RAM. > >> Performance starts out fine, and then the page cache fills up and > >> we > >> hit a throughput cliff. CPU usage of the IO threads go up, and we > >> have kswapd spending 100% of a core trying to keep up. Seeing > >> that, I was reminded of the many complaints I here about buffered > >> IO, and the fact that most of the folks complaining will > >> ultimately bite the bullet and move to O_DIRECT to just get the > >> kernel out of the way. > >> > >> But I don't think it needs to be like that. Switching to O_DIRECT > >> isn't always easily doable. The buffers have different life times, > >> size and alignment constraints, etc. On top of that, mixing > >> buffered > >> and O_DIRECT can be painful. > >> > >> Seems to me that we have an opportunity to provide something that > >> sits somewhere in between buffered and O_DIRECT, and this is where > >> RWF_UNCACHED enters the picture. If this flag is set on IO, we get > >> the following behavior: > >> > >> - If the data is in cache, it remains in cache and the copy (in or > >> out) is served to/from that. > >> > >> - If the data is NOT in cache, we add it while performing the IO. > >> When the IO is done, we remove it again. > >> > >> With this, I can do 100% smooth buffered reads or writes without > >> pushing the kernel to the state where kswapd is sweating bullets. > >> In > >> fact it doesn't even register. > > > > A question from a user or Linux Performance trainer perspective: > > > > How does this compare with posix_fadvise() with POSIX_FADV_DONTNEED > > that for example the nocache¹ command is using? Excerpt from > > manpage> > > posix_fadvice(2): > > POSIX_FADV_DONTNEED > > > > The specified data will not be accessed in the near > > future. > > > > POSIX_FADV_DONTNEED attempts to free cached pages as‐ > > sociated with the specified region. This is useful, > > for example, while streaming large files. A program > > may periodically request the kernel to free cached > > data that has already been used, so that more useful > > cached pages are not discarded instead. > > > > [1] packaged in Debian as nocache or available > > herehttps://github.com/ Feh/nocache > > > > In any way, would be nice to have some option in rsync… I still did > > not change my backup script to call rsync via nocache. > > I don't know the nocache tool, but I'm guessing it just does the > writes (or reads) and then uses FADV_DONTNEED to drop behind those > pages? That's fine for slower use cases, it won't work very well for > fast IO. The write side currently works pretty much like that > internally, whereas the read side doesn't use the page cache at all. Yes, it does that. And yeah I saw you changed the read site to bypass the cache entirely. Also as I understand it this is for asynchronous using io uring primarily? -- Martin
