Re: [patch 1/4 v6]swap: change block allocation algorithm for SSD

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 16 Jul 2013 04:43:20 +0800 Shaohua Li <shli@kernel.org> 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;
> +		}
> +	}
>
> ...
>

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