The patch titled
Subject: swap: add a simple detector for inappropriate swapin readahead
has been added to the -mm tree. Its filename is
swap-add-a-simple-detector-for-inappropriate-swapin-readahead.patch
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*** Remember to use Documentation/SubmitChecklist when testing your code ***
The -mm tree is included into linux-next and is updated
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------------------------------------------------------
From: Shaohua Li <shli@xxxxxxxxxx>
Subject: swap: add a simple detector for inappropriate swapin readahead
The swapin readahead does a blind readahead whether or not the swapin is
sequential. This is ok for harddisk because large reads have relatively
small costs and if the readahead pages are unneeded they can be reclaimed
easily. But for SSD devices large reads are more expensive than small
one. If readahead pages are unneeded, reading them in caused significant
overhead
This patch addes a simple random read detection similar to file mmap
readahead. If a 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 patch
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
significantly. Sequential write swap workload hasn't chanage.
Interestingly, the randwrite harddisk test is improved too. This might be
because swapin readahead needs to allocate extra memory, which further
tights memory pressure, so more swapout/swapin.
Signed-off-by: Shaohua Li <shli@xxxxxxxxxxxx>
Acked-by: Rik van Riel <riel@xxxxxxxxxx>
Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx>
Cc: Wu Fengguang <fengguang.wu@xxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Minchan Kim <minchan@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/rmap.h | 3 ++
mm/internal.h | 51 +++++++++++++++++++++++++++++++++++++++++
mm/memory.c | 3 +-
mm/rmap.c | 3 ++
mm/shmem.c | 1
mm/swap_state.c | 6 ++++
6 files changed, 66 insertions(+), 1 deletion(-)
diff -puN include/linux/rmap.h~swap-add-a-simple-detector-for-inappropriate-swapin-readahead include/linux/rmap.h
--- a/include/linux/rmap.h~swap-add-a-simple-detector-for-inappropriate-swapin-readahead
+++ a/include/linux/rmap.h
@@ -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 rb_root.rb_node is set by
diff -puN mm/internal.h~swap-add-a-simple-detector-for-inappropriate-swapin-readahead mm/internal.h
--- a/mm/internal.h~swap-add-a-simple-detector-for-inappropriate-swapin-readahead
+++ a/mm/internal.h
@@ -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);
@@ -357,4 +358,54 @@ extern unsigned long vm_mmap_pgoff(struc
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_THRESHOLD (100)
+#define SWAPRA_MAX_MISS ((SWAPRA_MISS_THRESHOLD) * 10)
+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_MAX_MISS)
+ 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_THRESHOLD;
+}
+#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 /* CONFIG_SWAP */
+
#endif /* __MM_INTERNAL_H */
diff -puN mm/memory.c~swap-add-a-simple-detector-for-inappropriate-swapin-readahead mm/memory.c
--- a/mm/memory.c~swap-add-a-simple-detector-for-inappropriate-swapin-readahead
+++ a/mm/memory.c
@@ -2968,7 +2968,8 @@ 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);
diff -puN mm/rmap.c~swap-add-a-simple-detector-for-inappropriate-swapin-readahead mm/rmap.c
--- a/mm/rmap.c~swap-add-a-simple-detector-for-inappropriate-swapin-readahead
+++ a/mm/rmap.c
@@ -366,6 +366,9 @@ static void anon_vma_ctor(void *data)
mutex_init(&anon_vma->mutex);
atomic_set(&anon_vma->refcount, 0);
+#ifdef CONFIG_SWAP
+ atomic_set(&anon_vma->swapra_miss, 0);
+#endif
anon_vma->rb_root = RB_ROOT;
}
diff -puN mm/shmem.c~swap-add-a-simple-detector-for-inappropriate-swapin-readahead mm/shmem.c
--- a/mm/shmem.c~swap-add-a-simple-detector-for-inappropriate-swapin-readahead
+++ a/mm/shmem.c
@@ -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;
return swapin_readahead(swap, gfp, &pvma, 0);
}
diff -puN mm/swap_state.c~swap-add-a-simple-detector-for-inappropriate-swapin-readahead mm/swap_state.c
--- a/mm/swap_state.c~swap-add-a-simple-detector-for-inappropriate-swapin-readahead
+++ a/mm/swap_state.c
@@ -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);
}
_
Patches currently in -mm which might be from shli@xxxxxxxxxx are
linux-next.patch
readahead-fault-retry-breaks-mmap-file-read-random-detection.patch
swap-add-a-simple-detector-for-inappropriate-swapin-readahead.patch
swap-add-a-simple-detector-for-inappropriate-swapin-readahead-fix.patch
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