On 03/06/2017 06:08 PM, Jens Axboe wrote:
On 03/06/2017 08:59 AM, Avi Kivity wrote:
On 03/06/2017 05:38 PM, Jens Axboe wrote:
On 03/06/2017 08:29 AM, Avi Kivity wrote:
On 03/06/2017 05:19 PM, Jens Axboe wrote:
On 03/06/2017 01:25 AM, Jan Kara wrote:
On Sun 05-03-17 16:56:21, Avi Kivity wrote:
The goal of the patch series is to return -EAGAIN/-EWOULDBLOCK if
any of these conditions are met. This way userspace can push most
of the write()s to the kernel to the best of its ability to complete
and if it returns -EAGAIN, can defer it to another thread.
Is it not possible to push the iocb to a workqueue? This will allow
existing userspace to work with the new functionality, unchanged. Any
userspace implementation would have to do the same thing, so it's not like
we're saving anything by pushing it there.
That is not easy because until IO is fully submitted, you need some parts
of the context of the process which submits the IO (e.g. memory mappings,
but possibly also other credentials). So you would need to somehow transfer
this information to the workqueue.
Outside of technical challenges, the API also needs to return EAGAIN or
start blocking at some point. We can't expose a direct connection to
queue work like that, and let any user potentially create millions of
pending work items (and IOs).
You wouldn't expect more concurrent events than the maxevents parameter
that was supplied to io_setup syscall; it should have reserved any
resources needed.
Doesn't matter what limit you apply, my point still stands - at some
point you have to return EAGAIN, or block. Returning EAGAIN without
the caller having flagged support for that change of behavior would
be problematic.
Doesn't it already return EAGAIN (or some other error) if you exceed
maxevents?
It's a setup thing. We check these limits when someone creates an IO
context, and carve out the specified entries form our global pool. Then
we free those "resources" when the io context is freed.
Right now I can setup an IO context with 1000 entries on it, yet that
number has NO bearing on when io_submit() would potentially block or
return EAGAIN.
We can have a huge gap on the intent signaled by io context setup, and
the reality imposed by what actually happens on the IO submission side.
Isn't that a bug? Shouldn't that 1001st incomplete io_submit() return
EAGAIN?
Just tested it, and maxevents is not respected for this:
io_setup(1, [0x7fc64537f000]) = 0
io_submit(0x7fc64537f000, 10, [{pread, fildes=3, buf=0x1eb4000,
nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096,
offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0},
{pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, {pread,
fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}, {pread, fildes=3,
buf=0x1eb4000, nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000,
nbytes=4096, offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096,
offset=0}, {pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0},
{pread, fildes=3, buf=0x1eb4000, nbytes=4096, offset=0}]) = 10
which is unexpected, to me.
