Re: [patch v2]swap: add a simple random read swapin detection

[Konstantin Khlebnikov <khlebnikov@openvz.org>]

Shaohua Li wrote:
> The swapin readahead does a blind readahead regardless if the swapin is
> sequential. This is ok for harddisk and random read, because read big size has
> no penality in harddisk, and if the readahead pages are garbage, they can be
> reclaimed fastly. But for SSD, big size read is more expensive than small size
> read. If readahead pages are garbage, such readahead only has overhead.
>
> This patch addes a simple random read detection like what file mmap readahead
> does. If random read is detected, swapin readahead will be skipped. This
> improves a lot for a swap workload with random IO in a fast SSD.
>
> I run anonymous mmap write micro benchmark, which will triger swapin/swapout.
> 			runtime changes with path
> randwrite harddisk	-38.7%
> seqwrite harddisk	-1.1%
> randwrite SSD		-46.9%
> seqwrite SSD		+0.3%

Very nice!

>
> For both harddisk and SSD, the randwrite swap workload run time is reduced
> significant. sequential write swap workload hasn't chanage.
>
> Interesting is the randwrite harddisk test is improved too. This might be
> because swapin readahead need allocate extra memory, which further tights
> memory pressure, so more swapout/swapin.
>
> This patch depends on readahead-fault-retry-breaks-mmap-file-read-random-detection.patch
>
> V1->V2:
> 1. Move the swap readahead accounting to separate functions as suggested by Riel.
> 2. Enable the logic only with CONFIG_SWAP enabled as suggested by Minchan.
>
> Signed-off-by: Shaohua Li<shli@fusionio.com>
> ---
>   include/linux/mm_types.h |    3 +++
>   mm/internal.h            |   44 ++++++++++++++++++++++++++++++++++++++++++++
>   mm/memory.c              |    3 ++-
>   mm/swap_state.c          |    8 ++++++++
>   4 files changed, 57 insertions(+), 1 deletion(-)
>

> --- linux.orig/include/linux/mm_types.h	2012-08-22 11:44:53.077912855 +0800
> +++ linux/include/linux/mm_types.h	2012-08-24 13:07:11.798576941 +0800
> @@ -279,6 +279,9 @@ struct vm_area_struct {
>   #ifdef CONFIG_NUMA
>   	struct mempolicy *vm_policy;	/* NUMA policy for the VMA */
>   #endif
> +#ifdef CONFIG_SWAP
> +	atomic_t swapra_miss;
> +#endif

You can place this atomic on vma->anon_vma, it has perfect 4-byte hole right 
after field "refcount". vma->anon_vma already exists since this vma already 
contains anon pages.

>   };
>
>   struct core_thread {

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