linux-kernel - Re: [PATCH 04/10] Use percpu buffers

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Message-ID: <4C60D9E6.3050700@vflare.org>
Date:	Tue, 10 Aug 2010 10:17:34 +0530
From:	Nitin Gupta <ngupta@...are.org>
To:	Pekka Enberg <penberg@...nel.org>
CC:	Pekka Enberg <penberg@...helsinki.fi>,
	Minchan Kim <minchan.kim@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <greg@...ah.com>,
	Linux Driver Project <devel@...uxdriverproject.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/10] Use percpu buffers

On 08/10/2010 12:27 AM, Pekka Enberg wrote:
> On Mon, Aug 9, 2010 at 8:26 PM, Nitin Gupta <ngupta@...are.org> wrote:
>> @@ -303,38 +307,41 @@ static int zram_write(struct zram *zram, struct bio *bio)
>>                                zram_test_flag(zram, index, ZRAM_ZERO))
>>                        zram_free_page(zram, index);
>>
>> -               mutex_lock(&zram->lock);
>> +               preempt_disable();
>> +               zbuffer = __get_cpu_var(compress_buffer);
>> +               zworkmem = __get_cpu_var(compress_workmem);
>> +               if (unlikely(!zbuffer || !zworkmem)) {
>> +                       preempt_enable();
>> +                       goto out;
>> +               }
> 
> The per-CPU buffer thing with this preempt_disable() trickery looks
> overkill to me. Most block device drivers seem to use mempool_alloc()
> for this sort of thing. Is there some reason you can't use that here?
> 

Other block drivers are allocating relatively small structs using
mempool_alloc(). However, in case of zram, these buffers are quite
large (compress_workmem is 64K!). So, allocating them on every write
would probably be much slower than using a pre-allocated per-cpu buffer.

Thanks,
Nitin
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