alt Hacker NewsSo I have a buffer pool with O_DIRECT reads.
I implement read-ahead in the application by (optionally) preadv:ing a single read into multiple destination buffers in the pool, leaving them unpinned, since as long as you aren't up against the bandwidth limit of the drive, a larger read is generally as fast as multiple smaller one on modern hardware.
I've tried doing this with io_uring as well, but found just eating the preadv syscall cost was faster.
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> a larger read is generally as fast as multiple smaller one on modern hardware.
Not always if by modern you mean NVMe drives. One synchronous preadv() for 256 KiB gives the kernel/device one big request but 16 independent asynchronous 16 KiB reads can be serviced concurrently. So the latter gives the NVMe controller 16 operations it can schedule in parallel. So depending on the workload and hardware, offsets, filesystem and request sizes that can give you lower aggregate latency or higher throughput.
Do this with io_uring with the preadv syscall. It’ll be the same or faster (faster only if you can do something else while waiting for I/O or you can submit multiple requests simultaneously - a single io_uring will be basically identical)