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

[Maxim Patlasov]

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.


> > > 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