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

Nikolaus Rath
2016-11-22T22:45:41Z

On Nov 16 2016, Maxim Patlasov <mpatlasov@virtuozzo.com> wrote:
> 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.

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).

Thanks!