From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx165.postini.com [74.125.245.165]) by kanga.kvack.org (Postfix) with SMTP id 104E16B0031 for ; Wed, 17 Jul 2013 18:00:08 -0400 (EDT) Date: Wed, 17 Jul 2013 15:00:07 -0700 From: Andrew Morton Subject: Re: [patch 1/4 v6]swap: change block allocation algorithm for SSD Message-Id: <20130717150007.ff10504603266dc221763315@linux-foundation.org> In-Reply-To: <20130715204320.GA7925@kernel.org> References: <20130715204320.GA7925@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Shaohua Li Cc: linux-mm@kvack.org, riel@redhat.com, minchan@kernel.org, kmpark@infradead.org, hughd@google.com, aquini@redhat.com On Tue, 16 Jul 2013 04:43:20 +0800 Shaohua Li wrote: > I'm using a fast SSD to do swap. scan_swap_map() sometimes uses up to 20~30% > CPU time (when cluster is hard to find, the CPU time can be up to 80%), which > becomes a bottleneck. scan_swap_map() scans a byte array to search a 256 page > cluster, which is very slow. > > Here I introduced a simple algorithm to search cluster. Since we only care > about 256 pages cluster, we can just use a counter to track if a cluster is > free. Every 256 pages use one int to store the counter. If the counter of a > cluster is 0, the cluster is free. All free clusters will be added to a list, > so searching cluster is very efficient. With this, scap_swap_map() overhead > disappears. > > This might help low end SD card swap too. Because if the cluster is aligned, SD > firmware can do flash erase more efficiently. > > We only enable the algorithm for SSD. Hard disk swap isn't fast enough and has > downside with the algorithm which might introduce regression (see below). > > The patch slightly changes which cluster is choosen. It always adds free > cluster to list tail. This can help wear leveling for low end SSD too. And if > no cluster found, the scan_swap_map() will do search from the end of last > cluster. So if no cluster found, the scan_swap_map() will do search from the > end of last free cluster, which is random. For SSD, this isn't a problem at > all. > > Another downside is the cluster must be aligned to 256 pages, which will reduce > the chance to find a cluster. I would expect this isn't a big problem for SSD > because of the non-seek penality. (And this is the reason I only enable the > algorithm for SSD). I have to agree with Will here - the patch adds a significant new design/algorithm into core MM but there wasn't even an attempt to describe it within the code. The changelog provdes a reasonable overview, most notably the second paragraph. Could you please find a way to flesh that part out a bit then integrate it into a code comment? And yes, the major functions should have their own comments explaining how they serve the overall scheme. > --- linux.orig/include/linux/swap.h 2013-07-11 19:14:36.849910383 +0800 > +++ linux/include/linux/swap.h 2013-07-11 19:14:38.657887654 +0800 > @@ -182,6 +182,17 @@ enum { > #define SWAP_MAP_SHMEM 0xbf /* Owned by shmem/tmpfs, in first swap_map */ > > /* > + * the data field stores next cluster if the cluster is free or cluster counter > + * otherwise > + */ > +struct swap_cluster_info { > + unsigned int data:24; > + unsigned int flags:8; > +}; If I'm understanding it correctly, the code and data structures which this patch adds are all protected by swap_info_struct.lock, yes? This is also worth mentioning in a comment, perhaps at the swap_cluster_info definition site > +#define CLUSTER_FLAG_FREE 1 /* This cluster is free */ > +#define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */ > > ... > > @@ -2117,13 +2311,28 @@ SYSCALL_DEFINE2(swapon, const char __use > error = -ENOMEM; > goto bad_swap; > } > + if (p->bdev && blk_queue_nonrot(bdev_get_queue(p->bdev))) { > + p->flags |= SWP_SOLIDSTATE; > + /* > + * select a random position to start with to help wear leveling > + * SSD > + */ > + p->cluster_next = 1 + (prandom_u32() % p->highest_bit); > + > + cluster_info = vzalloc(DIV_ROUND_UP(maxpages, > + SWAPFILE_CLUSTER) * sizeof(*cluster_info)); OK, what is the upper bound on the size of this allocation? A failure here would be bad - perhaps a list is needed, rather than a flat array. > + if (!cluster_info) { > + error = -ENOMEM; > + goto bad_swap; > + } > + } > > ... > -- 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