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SubjectRe: [patch] readv/writev rework

> > Your readv/writev patch interested me and I checked it.
> > I found we also have a chance to improve normal writev.
> >
> > a_ops->prepare_write() and a_ops->commit_write will have a
> > penalty when I/O size isn't PAGE_SIZE.
> > With following patch generic_file_write_nolock() will try to
> > make each I/O size become PAGE_SIZE.
> >
> Certainly makes a lot of sense. If an application has a large
> number of small objects which are to be appended to a file, and
> they are not contiguous in user memory then this patch makes
> writev() a very attractive way of doing that. Tons faster.

Yeah, I realized syslogd is using writev against logfiles which are
opened with O_SYNC flag! I think heavy loaded mail-servers or
web-servers may get good performance with the new writev
as they are logging too much.

And I'd also like to make knfsd use it as a recevied nfs request may
be splited into some IP fragments.

> However I'd be a little concerned over the increased work which a boring
> old write() has to do. Perhaps we could add a special code path
> for it:

It sounds nice.
I'll rewrite it soon.

> struct iovstate {
> unsigned rest;
> unsigned copy;
> unsigned off;
> struct iovec *work_iov;
> char *work_buf;
> unsigned work_iov_bytes;
> } iovstate;
> if (nr_segs != 1) {
> initialise iovstate;
> }
> ...
> if (nr_segs == 1)
> page_fault = filemap_copy_from_user(page, offset, buf, bytes);
> else
> page_fault = iov_copy_from_user(&arg1, &arg2, &bytes, &iovstate);
> status = a_ops->commit_write(file, page, offset, offset+bytes);
> then we can do all that fancy stuff in iov_copy_from_user(). That
> function should be inlined so we don't really pass all those arguments
> around. That's just a cleanliness thing, and branching around the complex
> code in the simple case. We should be able to get the impact on the
> common case down to a single taken branch per page.
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