On Nov 16 2016, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> wrote:
On 11/16/2016 12:19 PM, Nikolaus Rath wrote:What about buffered, asynchronous writes when writeback cache is
On Nov 16 2016, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> wrote:Surprise! Alas, Linux kernel does NOT process buffered AIO reads in
On 11/16/2016 11:19 AM, Nikolaus Rath wrote:Not sure I understand. What is it that's blocking? It can't be the
Hi Maxim,I think that's exhaustive list, but I can miss something.
On Nov 15 2016, Maxim Patlasov <mpatlasov@xxxxxxxxxxxxx> wrote:
On 11/15/2016 08:18 AM, Nikolaus Rath wrote:Ah, that makes sense. Are these two cases meant as examples, or is that
Could someone explain to me the meaning of the max_background andfuse uses max_background for cases where the total number of
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.
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.
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...?
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
userspace process, because then it wouldn't be asynchronous I/O...
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
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).