Re: [patch v3]swap: add a simple random read swapin detection

[Minchan Kim <minchan@kernel.org>]

On Thu, Aug 30, 2012 at 06:36:12PM +0800, Shaohua Li wrote:
> On Mon, Aug 27, 2012 at 06:52:07PM +0400, Konstantin Khlebnikov wrote:
> > >--- 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.
> 
> makes sense. vma->anon_vma could be NUll (shmem), but in shmem
> case, vma could NULL too, so maybe just ignore it.
> 
> 
> Subject: swap: add a simple random read swapin detection
> 
> 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%
> 
> 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
> 
> V2->V3:
> move swapra_miss to 'struct anon_vma' as suggested by Konstantin. 
> 
> 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/rmap.h |    3 +++
>  mm/internal.h        |   50 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  mm/memory.c          |    3 ++-
>  mm/shmem.c           |    1 +
>  mm/swap_state.c      |    6 ++++++
>  5 files changed, 62 insertions(+), 1 deletion(-)
> 
> Index: linux/mm/swap_state.c
> ===================================================================
> --- linux.orig/mm/swap_state.c	2012-08-29 16:13:00.912112140 +0800
> +++ linux/mm/swap_state.c	2012-08-30 18:28:24.678315187 +0800
> @@ -20,6 +20,7 @@
>  #include <linux/page_cgroup.h>
>  
>  #include <asm/pgtable.h>
> +#include "internal.h"
>  
>  /*
>   * swapper_space is a fiction, retained to simplify the path through
> @@ -379,6 +380,10 @@ struct page *swapin_readahead(swp_entry_
>  	unsigned long mask = (1UL << page_cluster) - 1;
>  	struct blk_plug plug;
>  
> +	swap_cache_miss(vma);
> +	if (swap_cache_skip_readahead(vma))
> +		goto skip;
> +
>  	/* Read a page_cluster sized and aligned cluster around offset. */
>  	start_offset = offset & ~mask;
>  	end_offset = offset | mask;
> @@ -397,5 +402,6 @@ struct page *swapin_readahead(swp_entry_
>  	blk_finish_plug(&plug);
>  
>  	lru_add_drain();	/* Push any new pages onto the LRU now */
> +skip:
>  	return read_swap_cache_async(entry, gfp_mask, vma, addr);
>  }
> Index: linux/mm/memory.c
> ===================================================================
> --- linux.orig/mm/memory.c	2012-08-29 16:13:00.920112040 +0800
> +++ linux/mm/memory.c	2012-08-30 13:32:05.425830660 +0800
> @@ -2953,7 +2953,8 @@ static int do_swap_page(struct mm_struct
>  		ret = VM_FAULT_HWPOISON;
>  		delayacct_clear_flag(DELAYACCT_PF_SWAPIN);
>  		goto out_release;
> -	}
> +	} else if (!(flags & FAULT_FLAG_TRIED))
> +		swap_cache_hit(vma);
>  
>  	locked = lock_page_or_retry(page, mm, flags);
>  
> Index: linux/mm/internal.h
> ===================================================================
> --- linux.orig/mm/internal.h	2012-08-29 16:13:00.932111888 +0800
> +++ linux/mm/internal.h	2012-08-30 18:28:03.698578951 +0800
> @@ -12,6 +12,7 @@
>  #define __MM_INTERNAL_H
>  
>  #include <linux/mm.h>
> +#include <linux/rmap.h>
>  
>  void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *start_vma,
>  		unsigned long floor, unsigned long ceiling);
> @@ -356,3 +357,52 @@ extern unsigned long vm_mmap_pgoff(struc
>          unsigned long, unsigned long);
>  
>  extern void set_pageblock_order(void);
> +
> +/*
> + * Unnecessary readahead harms performance. 1. for SSD, big size read is more
> + * expensive than small size read, so extra unnecessary read only has overhead.
> + * For harddisk, this overhead doesn't exist. 2. unnecessary readahead will
> + * allocate extra memroy, which further tights memory pressure, so more
> + * swapout/swapin.
> + * These adds a simple swap random access detection. In swap page fault, if
> + * page is found in swap cache, decrease an account of vma, otherwise we need
> + * do sync swapin and the account is increased. Optionally swapin will do
> + * readahead if the counter is below a threshold.
> + */
> +#ifdef CONFIG_SWAP
> +#define SWAPRA_MISS  (100)
> +static inline void swap_cache_hit(struct vm_area_struct *vma)
> +{
> +	if (vma && vma->anon_vma)
> +		atomic_dec_if_positive(&vma->anon_vma->swapra_miss);
> +}
> +
> +static inline void swap_cache_miss(struct vm_area_struct *vma)
> +{
> +	if (!vma || !vma->anon_vma)
> +		return;
> +	if (atomic_read(&vma->anon_vma->swapra_miss) < SWAPRA_MISS * 10)

You can use some meaningful macro instead of magic vaule 10.

/*
 * If swapra_miss is higher than SWAPRA_SKIP_THRESHOLD, swapin readahead
 * will be skipped.
 * swapra_miss count could be increased until SWAPRA_MISS_MAX_COUNT.
 * If swapra_miss count is decreased by SWAPRA_SKIP_THRESHOLD below
 * by cache hit, we can start swapin readahead, again.
 */
#define SWAPRA_SKIP_THRESHOLD 100
#define SWAPRA_MISS_MAX_COUNT		(SWAPRA_SKIP_THRESHOLD * 10)

> +		atomic_inc(&vma->anon_vma->swapra_miss);
> +}
> +
> +static inline int swap_cache_skip_readahead(struct vm_area_struct *vma)
> +{
> +	if (!vma || !vma->anon_vma)
> +		return 0;
> +	return atomic_read(&vma->anon_vma->swapra_miss) > SWAPRA_MISS;
> +}
> +#else
> +static inline void swap_cache_hit(struct vm_area_struct *vma)
> +{
> +}
> +
> +static inline void swap_cache_miss(struct vm_area_struct *vma)
> +{
> +}
> +
> +static inline int swap_cache_skip_readahead(struct vm_area_struct *vma)
> +{
> +	return 0;
> +}
> +
> +#endif
> Index: linux/include/linux/rmap.h
> ===================================================================
> --- linux.orig/include/linux/rmap.h	2012-06-01 10:10:31.686394463 +0800
> +++ linux/include/linux/rmap.h	2012-08-30 18:10:12.256048781 +0800
> @@ -35,6 +35,9 @@ struct anon_vma {
>  	 * anon_vma if they are the last user on release
>  	 */
>  	atomic_t refcount;
> +#ifdef CONFIG_SWAP
> +	atomic_t swapra_miss;
> +#endif
>  
>  	/*
>  	 * NOTE: the LSB of the head.next is set by
> Index: linux/mm/shmem.c
> ===================================================================
> --- linux.orig/mm/shmem.c	2012-08-06 16:00:45.465441525 +0800
> +++ linux/mm/shmem.c	2012-08-30 18:10:51.755553250 +0800
> @@ -933,6 +933,7 @@ static struct page *shmem_swapin(swp_ent
>  	pvma.vm_pgoff = index + info->vfs_inode.i_ino;
>  	pvma.vm_ops = NULL;
>  	pvma.vm_policy = spol;
> +	pvma.anon_vma = NULL;

So, shmem always do readahead blindly still?

>  	return swapin_readahead(swap, gfp, &pvma, 0);
>  }
>  

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