Hi Bernd, On 3/6/24 11:45 PM, Bernd Schubert wrote: > > > On 3/6/24 14:32, Jingbo Xu wrote: >> >> >> On 3/5/24 10:26 PM, Miklos Szeredi wrote: >>> On Mon, 26 Feb 2024 at 05:00, Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx> wrote: >>>> >>>> Hi Miklos, >>>> >>>> On 1/26/24 2:29 PM, Jingbo Xu wrote: >>>>> >>>>> >>>>> On 1/24/24 8:47 PM, Jingbo Xu wrote: >>>>>> >>>>>> >>>>>> On 1/24/24 8:23 PM, Miklos Szeredi wrote: >>>>>>> On Wed, 24 Jan 2024 at 08:05, Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx> wrote: >>>>>>>> >>>>>>>> From: Xu Ji <laoji.jx@xxxxxxxxxxxxxxx> >>>>>>>> >>>>>>>> Increase FUSE_MAX_MAX_PAGES limit, so that the maximum data size of a >>>>>>>> single request is increased. >>>>>>> >>>>>>> The only worry is about where this memory is getting accounted to. >>>>>>> This needs to be thought through, since the we are increasing the >>>>>>> possible memory that an unprivileged user is allowed to pin. >>>>> >>>>> Apart from the request size, the maximum number of background requests, >>>>> i.e. max_background (12 by default, and configurable by the fuse >>>>> daemon), also limits the size of the memory that an unprivileged user >>>>> can pin. But yes, it indeed increases the number proportionally by >>>>> increasing the maximum request size. >>>>> >>>>> >>>>>> >>>>>>> >>>>>>> >>>>>>> >>>>>>>> >>>>>>>> This optimizes the write performance especially when the optimal IO size >>>>>>>> of the backend store at the fuse daemon side is greater than the original >>>>>>>> maximum request size (i.e. 1MB with 256 FUSE_MAX_MAX_PAGES and >>>>>>>> 4096 PAGE_SIZE). >>>>>>>> >>>>>>>> Be noted that this only increases the upper limit of the maximum request >>>>>>>> size, while the real maximum request size relies on the FUSE_INIT >>>>>>>> negotiation with the fuse daemon. >>>>>>>> >>>>>>>> Signed-off-by: Xu Ji <laoji.jx@xxxxxxxxxxxxxxx> >>>>>>>> Signed-off-by: Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx> >>>>>>>> --- >>>>>>>> I'm not sure if 1024 is adequate for FUSE_MAX_MAX_PAGES, as the >>>>>>>> Bytedance floks seems to had increased the maximum request size to 8M >>>>>>>> and saw a ~20% performance boost. >>>>>>> >>>>>>> The 20% is against the 256 pages, I guess. >>>>>> >>>>>> Yeah I guess so. >>>>>> >>>>>> >>>>>>> It would be interesting to >>>>>>> see the how the number of pages per request affects performance and >>>>>>> why. >>>>>> >>>>>> To be honest, I'm not sure the root cause of the performance boost in >>>>>> bytedance's case. >>>>>> >>>>>> While in our internal use scenario, the optimal IO size of the backend >>>>>> store at the fuse server side is, e.g. 4MB, and thus if the maximum >>>>>> throughput can not be achieved with current 256 pages per request. IOW >>>>>> the backend store, e.g. a distributed parallel filesystem, get optimal >>>>>> performance when the data is aligned at 4MB boundary. I can ask my folk >>>>>> who implements the fuse server to give more background info and the >>>>>> exact performance statistics. >>>>> >>>>> Here are more details about our internal use case: >>>>> >>>>> We have a fuse server used in our internal cloud scenarios, while the >>>>> backend store is actually a distributed filesystem. That is, the fuse >>>>> server actually plays as the client of the remote distributed >>>>> filesystem. The fuse server forwards the fuse requests to the remote >>>>> backing store through network, while the remote distributed filesystem >>>>> handles the IO requests, e.g. process the data from/to the persistent store. >>>>> >>>>> Then it comes the details of the remote distributed filesystem when it >>>>> process the requested data with the persistent store. >>>>> >>>>> [1] The remote distributed filesystem uses, e.g. a 8+3 mode, EC >>>>> (ErasureCode), where each fixed sized user data is split and stored as 8 >>>>> data blocks plus 3 extra parity blocks. For example, with 512 bytes >>>>> block size, for each 4MB user data, it's split and stored as 8 (512 >>>>> bytes) data blocks with 3 (512 bytes) parity blocks. >>>>> >>>>> It also utilize the stripe technology to boost the performance, for >>>>> example, there are 8 data disks and 3 parity disks in the above 8+3 mode >>>>> example, in which each stripe consists of 8 data blocks and 3 parity >>>>> blocks. >>>>> >>>>> [2] To avoid data corruption on power off, the remote distributed >>>>> filesystem commit a O_SYNC write right away once a write (fuse) request >>>>> received. Since the EC described above, when the write fuse request is >>>>> not aligned on 4MB (the stripe size) boundary, say it's 1MB in size, the >>>>> other 3MB is read from the persistent store first, then compute the >>>>> extra 3 parity blocks with the complete 4MB stripe, and finally write >>>>> the 8 data blocks and 3 parity blocks down. >>>>> >>>>> >>>>> Thus the write amplification is un-neglectable and is the performance >>>>> bottleneck when the fuse request size is less than the stripe size. >>>>> >>>>> Here are some simple performance statistics with varying request size. >>>>> With 4MB stripe size, there's ~3x bandwidth improvement when the maximum >>>>> request size is increased from 256KB to 3.9MB, and another ~20% >>>>> improvement when the request size is increased to 4MB from 3.9MB. >>> >>> I sort of understand the issue, although my guess is that this could >>> be worked around in the client by coalescing writes. This could be >>> done by adding a small delay before sending a write request off to the >>> network. >>> >>> Would that work in your case? >> >> It's possible but I'm not sure. I've asked my colleagues who working on >> the fuse server and the backend store, though have not been replied yet. >> But I guess it's not as simple as increasing the maximum FUSE request >> size directly and thus more complexity gets involved. >> >> I can also understand the concern that this may increase the risk of >> pinning more memory footprint, and a more generic using scenario needs >> to be considered. I can make it a private patch for our internal product. >> >> Thanks for the suggestions and discussion. > > It also gets kind of solved in my fuse-over-io-uring branch - as long as > there are enough free ring entries. I'm going to add in a flag there > that other CQEs might be follow up requests. Really time to post a new > version. Thanks for the information. I've not read the fuse-over-io-uring branch yet, but sounds like it would be much helpful . Would there be a flag in the FUSE request indicating it's one of the linked FUSE requests? Is this feature, say linked FUSE requests, enabled only when io-uring is upon FUSE? -- Thanks, Jingbo
