On Fri, Feb 1, 2019 at 1:51 PM Xavi Hernandez <xhernan...@redhat.com> wrote:
> On Fri, Feb 1, 2019 at 1:25 PM Poornima Gurusiddaiah <pguru...@redhat.com> > wrote: > >> Can the threads be categorised to do certain kinds of fops? >> > > Could be, but creating multiple thread groups for different tasks is > generally bad because many times you end up with lots of idle threads which > waste resources and could increase contention. I think we should only > differentiate threads if it's absolutely necessary. > > >> Read/write affinitise to certain set of threads, the other metadata fops >> to other set of threads. So we limit the read/write threads and not the >> metadata threads? Also if aio is enabled in the backend the threads will >> not be blocked on disk IO right? >> > > If we don't block the thread but we don't prevent more requests to go to > the disk, then we'll probably have the same problem. Anyway, I'll try to > run some tests with AIO to see if anything changes. > I've run some simple tests with AIO enabled and results are not good. A simple dd takes >25% more time. Multiple parallel dd take 35% more time to complete. Xavi > All this is based on the assumption that large number of parallel read >> writes make the disk perf bad but not the large number of dentry and >> metadata ops. Is that true? >> > > It depends. If metadata is not cached, it's as bad as a read or write > since it requires a disk access (a clear example of this is the bad > performance of 'ls' in cold cache, which is basically metadata reads). In > fact, cached data reads are also very fast, and data writes could go to the > cache and be updated later in background, so I think the important point is > if things are cached or not, instead of if they are data or metadata. Since > we don't have this information from the user side, it's hard to tell what's > better. My opinion is that we shouldn't differentiate requests of > data/metadata. If metadata requests happen to be faster, then that thread > will be able to handle other requests immediately, which seems good enough. > > However there's one thing that I would do. I would differentiate reads > (data or metadata) from writes. Normally writes come from cached > information that is flushed to disk at some point, so this normally happens > in the background. But reads tend to be in foreground, meaning that someone > (user or application) is waiting for it. So I would give preference to > reads over writes. To do so effectively, we need to not saturate the > backend, otherwise when we need to send a read, it will still need to wait > for all pending requests to complete. If disks are not saturated, we can > have the answer to the read quite fast, and then continue processing the > remaining writes. > > Anyway, I may be wrong, since all these things depend on too many factors. > I haven't done any specific tests about this. It's more like a > brainstorming. As soon as I can I would like to experiment with this and > get some empirical data. > > Xavi > > >> Thanks, >> Poornima >> >> >> On Fri, Feb 1, 2019, 5:34 PM Emmanuel Dreyfus <m...@netbsd.org wrote: >> >>> On Thu, Jan 31, 2019 at 10:53:48PM -0800, Vijay Bellur wrote: >>> > Perhaps we could throttle both aspects - number of I/O requests per >>> disk >>> >>> While there it would be nice to detect and report a disk with lower than >>> peer performance: that happen sometimes when a disk is dying, and last >>> time I was hit by that performance problem, I had a hard time finding >>> the culprit. >>> >>> -- >>> Emmanuel Dreyfus >>> m...@netbsd.org >>> _______________________________________________ >>> Gluster-devel mailing list >>> Gluster-devel@gluster.org >>> https://lists.gluster.org/mailman/listinfo/gluster-devel >>> >>
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