On Wed, Feb 13, 2019 at 11:16 AM Manoj Pillai <mpil...@redhat.com> wrote:
> > > On Wed, Feb 13, 2019 at 10:51 AM Raghavendra Gowdappa <rgowd...@redhat.com> > wrote: > >> >> >> On Tue, Feb 12, 2019 at 5:38 PM Raghavendra Gowdappa <rgowd...@redhat.com> >> wrote: >> >>> All, >>> >>> We've found perf xlators io-cache and read-ahead not adding any >>> performance improvement. At best read-ahead is redundant due to kernel >>> read-ahead >>> >> >> One thing we are still figuring out is whether kernel read-ahead is >> tunable. From what we've explored, it _looks_ like (may not be entirely >> correct), ra is capped at 128KB. If that's the case, I am interested in few >> things: >> * Are there any realworld applications/usecases, which would benefit from >> larger read-ahead (Manoj says block devices can do ra of 4MB)? >> > > kernel read-ahead is adaptive but influenced by the read-ahead setting on > the block device (/sys/block/<dev>/queue/read_ahead_kb), which can be > tuned. For RHEL specifically, the default is 128KB (last I checked) but the > default RHEL tuned-profile, throughput-performance, bumps that up to 4MB. > It should be fairly easy to rig up a test where 4MB read-ahead on the > block device gives better performance than 128KB read-ahead. > Thanks Manoj. To add to what Manoj said and give more context here, Glusterfs being a fuse-based fs is not exposed as a block device. So, that's the first problem of where/how to tune and I've listed other problems earlier. > -- Manoj > > * Is the limit on kernel ra tunable a hard one? IOW, what does it take to >> make it to do higher ra? If its difficult, can glusterfs read-ahead provide >> the expected performance improvement for these applications that would >> benefit from aggressive ra (as glusterfs can support larger ra sizes)? >> >> I am still inclined to prefer kernel ra as I think its more intelligent >> and can identify more sequential patterns than Glusterfs read-ahead [1][2]. >> [1] https://www.kernel.org/doc/ols/2007/ols2007v2-pages-273-284.pdf >> [2] https://lwn.net/Articles/155510/ >> >> and at worst io-cache is degrading the performance for workloads that >>> doesn't involve re-read. Given that VFS already have both these >>> functionalities, I am proposing to have these two translators turned off by >>> default for native fuse mounts. >>> >>> For non-native fuse mounts like gfapi (NFS-ganesha/samba) we can have >>> these xlators on by having custom profiles. Comments? >>> >>> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1665029 >>> >>> regards, >>> Raghavendra >>> >>
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