Re: [PATCH 1/2] aio: add vectored I/O support

[Avi Kivity]

Joel Becker wrote:

> On Sat, Oct 16, 2004 at 07:18:45AM +0200, Avi Kivity wrote:
>  
>
> > 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'd think we should fix the submission path instead.  Why create
> iovs _and_ iocbs when we only need to create one?  And even if we
> decided aio_readv() was still nice to keep, we'd want to fix this
> inefficiency in io_submit().
>
>  
Using IO_CMD_READ for a vector entails

- converting the userspace structure (which might well an iovec) to iocbs
- copying all those iocbs to the kernel
- merging the iocbs
- generating multiple completions for the merged request
- copying the completions to userspace
- coalescing the multiple completions in userspace to a single completion

error handling is difficult as well. one would expect that a bad sector 
with multiple iocbs would only fail one of the requests. it seems to be 
non-trivial to implement this correctly.

IO_CMD_PREADV, by contrast, is very simple, intuitive, and efficient.

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

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