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SubjectRe: (disk/cpu) kernel performance problem
On Mon, Jul 26, 1999 at 11:10:28AM -0700, Gerald Aigner wrote:

> 1) The kernel copies data many times before it ends up in user
> space.

Err, not AIUI. The block device reads the data from the device into
bh->b_data. This is one copy, if the device doesn't do DMA.

If the request ultimately came from the page cache (as all file data reads
should / do), then bh->b_data will have been initialised to point to part of
the appropriate page in the page cache. If this was a bog-standard read() call,
the data then gets copied to user space.

So that's one or two copies, depending on whether you count reading from the
device as a copy.

> In other words, for read-only access it should not be necessary to
> touch the data read in if the user process is written carefully
> (i.e., reads page-aligned data into a page-aligned buffer).

It isn't necessary, if you've mmap()d it. If you've just read() it, there's
one copy to user space. IIRC there was a discussion about the whole "zero
copy" issue, I think the outcome was that this single copy to user space
was not significant performance-wise, anyway.

> After observing the above problem, I then started experimenting with
> mmap/mlock in the hope that mmap would give me better performance than
> simple read/write commands.

I'll have to look at this a bit more before I comment..

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