On 03/06/2017 10:25 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.
It's at least possible to pass the mm_struct to the workqueue, and I
imagine other process attributes. But I appreciate the difficulty.
It would be quite annoying to have to keep a large number of worker
threads active, just in case aio is not working. Modern NVMes have
fairly deep queues, and at the worst case, you'll need one thread for
each I/O to keep everything busy.
