On 06/15/2017 01:25 PM, Andrew Morton wrote:
On Thu, 15 Jun 2017 10:59:52 -0500 Goldwyn Rodrigues [off-list ref] wrote:
quoted
This series adds nonblocking feature to asynchronous I/O writes.
io_submit() can be delayed because of a number of reason:
- Block allocation for files
- Data writebacks for direct I/O
- Sleeping because of waiting to acquire i_rwsem
- Congested block device
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.
In order to enable this, IOCB_RW_FLAG_NOWAIT is introduced in
uapi/linux/aio_abi.h. If set for aio_rw_flags, it translates to
IOCB_NOWAIT for struct iocb, REQ_NOWAIT for bio.bi_opf and IOMAP_NOWAIT for
iomap. aio_rw_flags is a new flag replacing aio_reserved1. We could
not use aio_flags because it is not currently checked for invalidity
in the kernel.
This feature is provided for direct I/O of asynchronous I/O only. I have
tested it against xfs, ext4, and btrfs while I intend to add more filesystems.
The nowait feature is for request based devices. In the future, I intend to
add support to stacked devices such as md.
Applications will have to check supportability by sending a async direct write
and any other error besides -EAGAIN would mean it is not supported.
How accurate it this? For example, the changes to
generic_file_direct_write() appear to greatly reduce the chances of
blocking but there are surely race opportunities which will still
result in userspace unexpectedly experiencing blocking in a succeednig
write() call?
We are not reducing the chance of blocking, but detecting if the call
would block and return to userspace as soon as possible rather than
waiting for the blocking factor. One of the blocking factor is the mutex
inode->i_rwsem (formerly i_mutex). The performance gain should come from
the application depending on how they use it. Here is an example:
A database application has compute and I/O threads. This effort will
allow the compute threads to push writes without the need of context
switch to I/O thread, since it knows that it will end soon enough
without blocking. If a IOCB does block (and returns -EAGAIN), it would
be deferred to the I/O thread. Usually the compute thread should know
the offsets of writes, and be careful not to overwrite other writes.
If correct then I think there should be some discussion and perhaps
testing results in the changelog.
I will be posting one test case to xfstests.
I have only minor quibbles - I'll grab the patch series for some -next
testing (at least).
I agree to the quibbles you have on patch 02/10. Should I send the
entire fixed series, just the 02/10 patch, or would you prefer to fix it?
--
Goldwyn