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

Maxim Patlasov
2016-11-16T22:14:19Z

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.

>
>>> 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