问题
I'm using the Linux kernel AIO through libaio, and I have to submit the next reading operation before the previous one was completed. The problem is that io_submit() blocks for some time and, as I can deduce from the interval, it waits the previous operation to be completed.
I know that I can enqueue a several operations with a single io_submit(), but it is not an option for me, because I don't know how exactly the next read operation would like when it's already a time to submit the first one.
Is it working like that only for me, or for everyone? In the second case, may I ask if I'm looking for something feasible, or I have to fallback to a threaded model?
回答1:
Frustratingly there are lots of reasons why io_submit may block including:
- You're doing buffered I/O
- You're doing I/O into a filesystem and your submission is queued behind a synchronous operation.
It's known that ext4 and AIO may not be the best mix:
Blocking during io_submit on ext4, on buffered operations, network access, pipes, etc. [...] AIO access to a file on a filesystem like ext4 is partially supported: if a metadata read is required to look up the data block (ie if the metadata is not already in memory), then the io_submit call will block on the metadata read. Certain types of file-enlarging writes are completely unsupported and block for the entire duration of the operation.
(extract is from a document that was called AIOUserGuide)
See the asynchronous IO io_submit latency in Ubuntu Linux question for other detailed answers.
来源：https://stackoverflow.com/questions/31175303/io-submit-blocks-until-a-previous-operation-will-be-completed