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

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Message-ID: <4C60E48A.5090608@vflare.org>
Date:	Tue, 10 Aug 2010 11:02:58 +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@...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,

On 08/10/2010 10:35 AM, Pekka Enberg wrote:
> 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?
> 

mempool_alloc() first calls alloc_fn with ~(__GFP_WAIT | __GFP_IO)
and *then* falls down to pre-allocated buffers. So, it will always
be slower than directly using pre-allocated buffers as is done
currently.

One trick we can use is to have alloc_fn such that it always returns
failure with ~__GFP_WAIT and do actual allocation otherwise. But still
it seems like unnecessary cost.

Thanks,
Nitin
 


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