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

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Date:	Tue, 10 Aug 2010 08:05:18 +0300
From:	Pekka Enberg <penberg@...nel.org>
To:	ngupta@...are.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@...verdev.osuosl.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 04/10] Use percpu buffers

  Hi Nitin,

On 10.8.2010 7.47, Nitin Gupta wrote:
> 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.
The mempool API is precisely for that - using pre-allocated buffers 
instead of allocating every time. The preempt_disable() games make the 
code complex and have the downside of higher scheduling latencies so why 
not give mempools a try?

             Pekka
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