From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: From: Nikolaus Rath To: Maxim Patlasov Cc: linux-kernel , Miklos Szeredi , , linux-fsdevel Subject: Re: [fuse-devel] fuse: max_background and congestion_threshold settings References: <87oa1g90nx.fsf@thinkpad.rath.org> <64a57faa-d3a6-a209-8728-723ed7f37c2f@virtuozzo.com> <87fumrmdvn.fsf@thinkpad.rath.org> <716677ab-f962-1628-205b-2326219f4487@virtuozzo.com> Date: Wed, 16 Nov 2016 12:19:52 -0800 In-Reply-To: <716677ab-f962-1628-205b-2326219f4487@virtuozzo.com> (Maxim Patlasov's message of "Wed, 16 Nov 2016 11:56:10 -0800") Message-ID: <877f83mb2v.fsf@thinkpad.rath.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Sender: linux-kernel-owner@vger.kernel.org List-ID: On Nov 16 2016, Maxim Patlasov wrote: > On 11/16/2016 11:19 AM, Nikolaus Rath wrote: > >> Hi Maxim, >> >> On Nov 15 2016, Maxim Patlasov 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=3D=3D8= 192 >>> 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... >> 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? Thanks, -Nikolaus --=20 GPG encrypted emails preferred. Key id: 0xD113FCAC3C4E599F Fingerprint: ED31 791B 2C5C 1613 AF38 8B8A D113 FCAC 3C4E 599F =C2=BBTime flies like an arrow, fruit flies like a Banana.=C2= =AB