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

Nikolaus Rath
2016-11-16T19:19:31Z

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...?

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?

Best,