Re: [fuse-devel] fuse: max_background and congestion_threshold settings

[Maxim Patlasov]

On 11/22/2016 02:45 PM, Nikolaus Rath wrote:

> On Nov 16 2016, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> wrote:
> > On 11/16/2016 12:19 PM, Nikolaus Rath wrote:
> >
> > > On Nov 16 2016, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> wrote:
> > > > On 11/16/2016 11:19 AM, Nikolaus Rath wrote:
> > > >
> > > > > Hi Maxim,
> > > > >
> > > > > On Nov 15 2016, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> 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.
> What about buffered, asynchronous writes when writeback cache is
> disabled? It sounds as if io_submit does not block (so userspace could
> create an unlimited number), nor can the kernel coalesce them (since
> writeback caching is disabled).

I've never looked closely at it. Do you have a particular use case or 
concern?


> Thanks!
> -Nikolaus