Zero Copy IO

Zero Copy IO

[Alex Q Chen]

I am trying to find a way to pin down user space memory from kernel, so
that these user space buffer can be used for direct IO transfer or
otherwise known as "zero copying IO".  Searching through the Internet and
reading comments on various news groups, it would appear that most
developers including Linus himself doesn't believe in the benefit of "zero
copying IO".  Most of the discussion however was based on network card
drivers.  For certain other drivers such as SCSI Tape driver, which need to
handle great deal of data transfer, it would seemed still be more
advantageous to enable zero copy IO than copy_from_user() and copy_to_user
() all the data.  Other OS such as AIX and OS2 have kernel functions that
can be used to accomplish such a task.  Has any ground work been done in
Linux 2.4 to enable "zero copying IO"?

Thanks in advance for any suggestions or comments

Sincerely,
Alex Chen

IBM SSD Device Driver Development
Office: 9000 S. Rita Rd 9032/2262
Email: aqchen@us.ibm.com
Phone: (external) 520-799-5212 (Tie Line) (321)-5212

Re: Zero Copy IO

[Andi Kleen]

On Sun, Apr 08, 2001 at 04:31:27PM -0700, Alex Q Chen wrote:
> I am trying to find a way to pin down user space memory from kernel, so
> that these user space buffer can be used for direct IO transfer or
> otherwise known as "zero copying IO".  Searching through the Internet and
> reading comments on various news groups, it would appear that most
> developers including Linus himself doesn't believe in the benefit of "zero
> copying IO".  Most of the discussion however was based on network card
> drivers.  For certain other drivers such as SCSI Tape driver, which need to
> handle great deal of data transfer, it would seemed still be more
> advantageous to enable zero copy IO than copy_from_user() and copy_to_user
> () all the data.  Other OS such as AIX and OS2 have kernel functions that
> can be used to accomplish such a task.  Has any ground work been done in
> Linux 2.4 to enable "zero copying IO"?

Yes, e.g. the raw io device does it using kiovecs. See 
drivers/char/raw.c,fs/iobuf.c et.al. 2.4+zerocopy networking also has a 
different implementation.
The raw.c implementation is not very efficient at the moment though,
mostly because of limitations in the block device layer (but that 
should be no problem for a direct tape driver) 
This work is also in the 2.2 kernels of most distributions.


-Andi

Re: Zero Copy IO

[Alan Cox]

> advantageous to enable zero copy IO than copy_from_user() and copy_to_user
> () all the data.  Other OS such as AIX and OS2 have kernel functions that
> can be used to accomplish such a task.  Has any ground work been done in
> Linux 2.4 to enable "zero copying IO"?

kiovecs support this. Note that the current kiovec has problems when it comes
to certain kinds of latency critical use and the 2.5 kernel meeting hashed out
some big changes here. But for 2.4 the kiovecs are there 

Re: Zero Copy IO

[Jeremy Jackson]

Douglas Gilbert wrote:

> "Alex Q Chen" <aqchen@us.ibm.com> wrote:
>
> > I am trying to find a way to pin down user space
> > memory from kernel, so that these user space buffer
> > can be used for direct IO transfer or otherwise
> > known as "zero copying IO".  Searching through the
> > Internet and reading comments on various news groups,
> > it would appear that most developers including Linus
> > himself doesn't believe in the benefit of "zero
> > copying IO".  Most of the discussion however was based
> > on network card drivers.  For certain other drivers
> > such as SCSI Tape driver, which need to handle great
> > deal of data transfer, it would seemed still be more
> > advantageous to enable zero copy IO than copy_from_user()
> > and copy_to_user() all the data.  Other OS such as AIX
> > and OS2 have kernel functions that can be used to
> > accomplish such a task.  Has any ground work been done
> > in Linux 2.4 to enable "zero copying IO"?
>
> Alex,
> The kiobufs mechanism in the 2.4 series is the appropriate
> tool for avoiding copy_from_user() and copy_to_user().
> The definitive driver is in drivers/char/raw.c which
> does synchronous IO to block devices such as disks
> (but is probably not appropriate for tapes).
>
> The SCSI generic (sg) driver supports direct IO. The driver
> in lk 2.4.3 has the direct IO code commented out while
> a version that I'm currently testing (sg 3.1.18 at
> www.torque.net/sg) has its direct IO code activated. I have
> a web page comparing throughput times and CPU utilizations
> at http://www.torque.net/sg/rbuf_tbl.html . My testing
> indicates that the kiobufs mechanism is now working
> quite well. For various reasons I still think that it
> is best to default to indirect IO and let speed hungry
> users enable dio (which is done in sg via procfs). Even
> when the user selects direct IO is should be possible to
> fall back to indirect IO. [Sg does this when a SCSI
> adapter can't support direct IO (e.g. an ISA adapter).]
>
> Since the SCSI tape (st) driver is structurally similar
> to sg, it should be possible to add direct IO support
> to st.
>
> One thing to note is that when you let the user provide
> the buffer for direct IO (e.g. with malloc) then on
> the i386 it won't be contiguous from a bus address POV.
> This means large scatter gather lists (typically with
> each element 4 KB on i386) which can be time consuming
> to load on some SCSI adapters. One way around this would
> be for a driver to provide "malloc/free" like ioctls.

I'm not very knowledgeable, but doesn't the sound driver
use mmap() to do this?  Either way,  AGP GART is
basically a paged-MMU allowing non-contiguous phys-mem
to be made to look contiguous from the AGP *and* from PCI
(on most chipsets?).  perhaps this would be helpful.

Large contiguous phys-mem seems to be difficult currently,
but would be nice as it would allow use of larger MMU pages
with many cpus.  Someone mentioned reverse page table support
would be required first...

>
> Doug Gilbert
> -
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Re: Zero Copy IO

[Douglas Gilbert]

"Alex Q Chen" <aqchen@us.ibm.com> wrote:

> I am trying to find a way to pin down user space 
> memory from kernel, so that these user space buffer 
> can be used for direct IO transfer or otherwise 
> known as "zero copying IO".  Searching through the 
> Internet and reading comments on various news groups, 
> it would appear that most developers including Linus 
> himself doesn't believe in the benefit of "zero 
> copying IO".  Most of the discussion however was based 
> on network card drivers.  For certain other drivers 
> such as SCSI Tape driver, which need to handle great 
> deal of data transfer, it would seemed still be more
> advantageous to enable zero copy IO than copy_from_user() 
> and copy_to_user() all the data.  Other OS such as AIX 
> and OS2 have kernel functions that can be used to 
> accomplish such a task.  Has any ground work been done 
> in Linux 2.4 to enable "zero copying IO"?

Alex,
The kiobufs mechanism in the 2.4 series is the appropriate
tool for avoiding copy_from_user() and copy_to_user().
The definitive driver is in drivers/char/raw.c which
does synchronous IO to block devices such as disks
(but is probably not appropriate for tapes).

The SCSI generic (sg) driver supports direct IO. The driver
in lk 2.4.3 has the direct IO code commented out while
a version that I'm currently testing (sg 3.1.18 at
www.torque.net/sg) has its direct IO code activated. I have
a web page comparing throughput times and CPU utilizations
at http://www.torque.net/sg/rbuf_tbl.html . My testing
indicates that the kiobufs mechanism is now working
quite well. For various reasons I still think that it 
is best to default to indirect IO and let speed hungry
users enable dio (which is done in sg via procfs). Even 
when the user selects direct IO is should be possible to
fall back to indirect IO. [Sg does this when a SCSI
adapter can't support direct IO (e.g. an ISA adapter).]

Since the SCSI tape (st) driver is structurally similar 
to sg, it should be possible to add direct IO support 
to st.

One thing to note is that when you let the user provide
the buffer for direct IO (e.g. with malloc) then on
the i386 it won't be contiguous from a bus address POV.
This means large scatter gather lists (typically with
each element 4 KB on i386) which can be time consuming
to load on some SCSI adapters. One way around this would
be for a driver to provide "malloc/free" like ioctls.

Doug Gilbert