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

Maxim Patlasov
2016-11-23T04:35:05Z

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

> 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).

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

>
> Thanks!
> -Nikolaus
>