RE: Expense of read_iter

[David Laight]

From: Mikulas Patocka
> Sent: 20 January 2021 15:12
> 
> On Wed, 20 Jan 2021, Jan Kara wrote:
> 
> > Yeah, I agree. I'm against ext4 private solution for this read problem. And
> > I'm also against duplicating ->read_iter functionatily in ->read handler.
> > The maintenance burden of this code duplication is IMHO just too big. We
> > rather need to improve the generic code so that the fast path is faster.
> > And every filesystem will benefit because this is not ext4 specific
> > problem.
> >
> > 								Honza
> 
> Do you have some idea how to optimize the generic code that calls
> ->read_iter?
> 
> vfs_read calls ->read if it is present. If not, it calls new_sync_read.
> new_sync_read's frame size is 128 bytes - it holds the structures iovec,
> kiocb and iov_iter. new_sync_read calls ->read_iter.
> 
> I have found out that the cost of calling new_sync_read is 3.3%, Zhongwei
> found out 3.9%. (the benchmark repeatedy reads the same 4k page)
> 
> I don't see any way how to optimize new_sync_read or how to reduce its
> frame size. Do you?

Why is the 'read_iter' path not just the same as the 'read' one
but calling copy_to_iter() instead of copy_to_user().

For a single fragment iov[] the difference might just be
measurable for a single byte read.
But by the time you are transferring 4k it ought to be miniscule.

It isn't as though you have the cost of reading the iov[] from userspace.
(That hits sendmsg() v send().)

	David

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