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SubjectRe: [PATCH 1/2] aio: add vectored I/O support
Joel Becker wrote:

>On Thu, Oct 14, 2004 at 01:10:01PM -0700, Yasushi Saito wrote:
>
>
>>This is a patch against 2.6.9-rc3-mm3 that add supports for vectored
>>async I/O. It adds two additional commands, IO_CMD_PREADV and
>>IO_CMD_PWRITEV to libaio.h. The below is roughly what I did:
>>
>>
>
> How does this differ substantially from lio_listio() of each I/O
>range? Does it have some significant performance win, or is it just
>aiming for a completeness that POSIX doesn't (to my knowledge) specify?
>
>
>
It is a huge performance win, at least on the 2.4-based RHEL kernel.
Large reads (~256K) using 4K iocbs are very slow on a large RAID, while
after I coded a similar patch I got a substantial speedup.

I don't know the cause for the slowdown; maybe request merging only
works for queued requests, and as the RAID has a large TCQ depth,
requests didn't have much of a chance to queue in the kernel.


--
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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