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SubjectRe: [PATCH] 2.5.21 Nonlinear CPU support
At 18:36 12/06/02, Andreas Dilger wrote:
>On Jun 12, 2002 16:08 +0100, Anton Altaparmakov wrote:
> > Buffer allocation at use time is NOT an option because the buffers are
> > allocated using vmalloc() which is extremely expensive and we would
> need to
> > allocate at every single initial ->readpage() call of a compressed file.
> >
> > >CPU hotswap higlights the fact that per CPU allocation needs to be smarter
> > >about doing its job (i.e. don't allocate if it won't be used ever,
> > >defer allocation to CPU hotswap event).
> >
> > The former is not possible for ntfs as there is no quick way to tell if
> use
> > will use decompression or not. And the latter creates a lot of complexity.
> > I gave an example using a callback of how it could be done in a previous
> > post but I don't like introducing complexity for a minority group of users.
>
>I think the reasonable solution is as follows:
>1) Allocate an array of NULL pointers which is NR_CPUs in size (you could do
> this all the time, as it would only be a few bytes)

Yes, that is fine.

>2) If you need to do decompression on a cpu you check the array entry
> for that CPU and if is NULL you vmalloc() the decompression buffers once
> for that CPU. This avoid vmalloc() overhead for each read.

The vmalloc() sleeps and by the time you get control back you are executing
on a different CPU. Ooops. The only valid way of treating per-cpu data is:

- disable preemption
- get the cpu number = START OF CRITICAL SECTION: no sleep/schedule allowed
- do work using the cpu number
- reenable preemption = END OF CRITICAL SECTION

The only thing that could possibly be used inside the critical region is
kmalloc(GFP_ATOMIC) but we are allocating 64kiB so that is not an option.
(It would fail very quickly due to memory fragmentation, the order of the
allocation is too high.)

>3) Any allocated buffers are freed in the same manner they are now -
> when the last compressed volume is unmounted. There may be some or
> all entries that are still NULL.
>
>This also avoids allocating buffers when there are no files which are
>actually compressed.

True it does, but unfortunately it doesn't work. )-:

Best regards,

Anton


--
"I've not lost my mind. It's backed up on tape somewhere." - Unknown
--
Anton Altaparmakov <aia21 at cantab.net> (replace at with @)
Linux NTFS Maintainer / IRC: #ntfs on irc.openprojects.net
WWW: http://linux-ntfs.sf.net/ & http://www-stu.christs.cam.ac.uk/~aia21/

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