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...? 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, -Nikolaus -- GPG encrypted emails preferred. Key id: 0xD113FCAC3C4E599F Fingerprint: ED31 791B 2C5C 1613 AF38 8B8A D113 FCAC 3C4E 599F »Time flies like an arrow, fruit flies like a Banana.« -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html