Messages in this thread | | | Subject | Re: [fuse-devel] fuse: max_background and congestion_threshold settings | From | Maxim Patlasov <> | Date | Wed, 16 Nov 2016 12:41:03 -0800 |
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On 11/16/2016 12:19 PM, Nikolaus Rath wrote:
> On Nov 16 2016, Maxim Patlasov <mpatlasov@virtuozzo.com> wrote: >> On 11/16/2016 11:19 AM, Nikolaus Rath wrote: >> >>> Hi Maxim, >>> >>> On Nov 15 2016, Maxim Patlasov <mpatlasov@virtuozzo.com> wrote: >>>> On 11/15/2016 08:18 AM, Nikolaus Rath wrote: >>>>> Could someone explain to me the meaning of the max_background and >>>>> congestion_threshold settings of the fuse module? >>>>> >>>>> At first I assumed that max_background specifies the maximum number of >>>>> pending requests (i.e., requests that have been send to userspace but >>>>> for which no reply was received yet). But looking at fs/fuse/dev.c, it >>>>> looks as if not every request is included in this number. >>>> fuse uses max_background for cases where the total number of >>>> simultaneous requests of given type is not limited by some other >>>> natural means. AFAIU, these cases are: 1) async processing of direct >>>> IO; 2) read-ahead. As an example of "natural" limitation: when >>>> userspace process blocks on a sync direct IO read/write, the number of >>>> requests fuse consumed is limited by the number of such processes >>>> (actually their threads). In contrast, if userspace requests 1GB >>>> direct IO read/write, it would be unreasonable to issue 1GB/128K==8192 >>>> fuse requests simultaneously. That's where max_background steps in. >>> Ah, that makes sense. Are these two cases meant as examples, or is that >>> an exhaustive list? Because I would have thought that other cases should >>> be writing of cached data (when writeback caching is enabled), and >>> asynchronous I/O from userspace...? >> I think that's exhaustive list, but I can miss something. >> >> As for writing of cached data, that definitely doesn't go through >> background requests. Here we rely on flusher: fuse will allocate as >> many requests as the flusher wants to writeback. >> >> Buffered AIO READs actually block in submit_io until fully >> processed. So it's just another example of "natural" limitation I told >> above. > Not sure I understand. What is it that's blocking? It can't be the > userspace process, because then it wouldn't be asynchronous I/O...
Surprise! Alas, Linux kernel does NOT process buffered AIO reads in async manner. You can verify it yourself by strace-ing a simple program looping over io_submit + io_getevents: for direct IO (as expected) io_submit returns immediately while io_getevents waits for actual IO; in contrast, for buffered IO (surprisingly) io_submit waits for actual IO while io_getevents returns immediately. Presumably, people are supposed to use mmap-ed read/writes rather than buffered AIO.
> >>> Also, I am not sure what you mean with async processing of direct >>> I/O. Shouldn't direct I/O always go directly to the file-system? If so, >>> how can it be processed asynchronously? >> That's a nice optimization we implemented a few years ago: having >> incoming sync direct IO request of 1MB size, kernel fuse splits it >> into eight 128K requests and starts processing them in async manner, >> waiting for the completion of all of them before completing that >> incoming 1MB requests. > I see. But why isn't that also done for regular (non-direct) IO?
Regular READs are helped by async read-ahead. Regular writes go through writeback mechanics: flusher calls fuse_writepages() and the latter submits as many async write requests as needed. Everything looks fine. (but as I wrote those async requests are not under fuse max_backgroung control).
Thanks, Maxim
> > Thanks, > -Nikolaus
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