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Date: Wed, 12 Jun 2002 15:03:56 -0600
From: Andreas Dilger <adilger@clusterfs.com>
To: Anton Altaparmakov <aia21@cantab.net>
Cc: vda@port.imtp.ilyichevsk.odessa.ua, Rusty Russell <rusty@rustcorp.com.au>,
        torvalds@transmeta.com, linux-kernel@vger.kernel.org,
        k-suganuma@mvj.biglobe.ne.jp, Andrew Morton <akpm@zip.com.au>
Subject: Re: [PATCH] 2.5.21 Nonlinear CPU support
Message-ID: <20020612210356.GD682@clusterfs.com>
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	vda@port.imtp.ilyichevsk.odessa.ua,
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On Jun 12, 2002  21:41 +0100, Anton Altaparmakov wrote:
> At 20:39 12/06/02, Andreas Dilger wrote:
> >At most you
> >will need to loop once for each available CPU if you are unlucky enough
> >to be rescheduled to a different CPU after each call to vmalloc().
> 
> Um are you suggesting compression buffers to be per mounted volume? That 
> would be more wasteful than the current approach of one buffer per CPU 
> globally for all of ntfs driver.

No, my mistake.  You should check whatever array you want.

> vfree() at a guess (I may be completely wrong on that one in which case I 
> appologize!) can also sleep so that breaks that scheme.

Well, just get rid of the while loop then and use an if+goto for both
the vmalloc and the vfree case.  At most we can loop NR_CPUS times.


	char *newbuf = NULL;
	int cpunum;
       
recheck:
	cpunum = current_cpu();
	if (unlikely(ntfs_compr_array[cpunum] == NULL)) {
		newbuf = vmalloc(NTFS_DECOMPR_BUFFER_SIZE);

		/*
		 * Re-check the buffer case we slept in vmalloc() and
		 * someone else already allocated a buffer for "this" CPU.
		 */
		if (likely(ntfs_compr_array[cpunum] == NULL)) {
			ntfs_compr_array[cpunum] = newbuf;
			newbuf = NULL;
		}
		goto recheck;
	}
	/* Hmm, we slept in vmalloc and we don't need the new buffer */
	if (unlikely(newbuf != NULL)) {
		vfree(newbuf);
		goto recheck;
	}

> But if doing something like that I might as well use the present 
> approach and just allocate all buffers at once if they haven't been 
> allocated yet and be done with it. Then no vfree()s are needed either and 
> then it really does work. (-;

But then you may be allocating a lot of memory for CPUs that don't
even exist, which is the whole point of this exercise.  Better to do
it on-demand and loop for the very few times needed.

Cheers, Andreas
--
Andreas Dilger
http://www-mddsp.enel.ucalgary.ca/People/adilger/
http://sourceforge.net/projects/ext2resize/

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