Messages in this thread | | | Date | Wed, 12 Jun 2002 19:34:39 +0100 | From | Anton Altaparmakov <> | Subject | Re: [PATCH] 2.5.21 Nonlinear CPU support |
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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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