From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail203.messagelabs.com (mail203.messagelabs.com [216.82.254.243]) by kanga.kvack.org (Postfix) with SMTP id A5222600044 for ; Tue, 10 Aug 2010 01:32:33 -0400 (EDT) Received: by ywg8 with SMTP id 8so4772736ywg.14 for ; Mon, 09 Aug 2010 22:32:32 -0700 (PDT) Message-ID: <4C60E48A.5090608@vflare.org> Date: Tue, 10 Aug 2010 11:02:58 +0530 From: Nitin Gupta Reply-To: ngupta@vflare.org MIME-Version: 1.0 Subject: Re: [PATCH 04/10] Use percpu buffers References: <1281374816-904-1-git-send-email-ngupta@vflare.org> <1281374816-904-5-git-send-email-ngupta@vflare.org> <4C60D9E6.3050700@vflare.org> <4C60DE0E.2000707@kernel.org> In-Reply-To: <4C60DE0E.2000707@kernel.org> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org To: Pekka Enberg Cc: Pekka Enberg , Minchan Kim , Andrew Morton , Greg KH , Linux Driver Project , linux-mm , linux-kernel List-ID: 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 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org