From: Christian Ehrhardt <ehrhardt@xxxxxxxxxxxxxxxxxx>
Swap readahead works fine, but the I/O to disk is almost always done in page
size requests, despite the fact that readahead submits 1<<page-cluster pages
at a time.
On older kernels the old per device plugging behavior might have captured
this and merged the requests, but currently all comes down to much more I/Os
than required.
On a single device this might not be an issue, but as soon as a server runs
on shared san resources savin I/Os not only improves swapin throughput but
also provides a lower resource utilization.
With a load running KVM in a lot of memory overcommitment (the hot memory
is 1.5 times the host memory) swapping throughput improves significantly
and the lead feels more responsive as well as achieves more throughput.
In a test setup with 16 swap disks running blocktrace on one of those disks
shows the improved merging:
Prior:
Reads Queued: 560,888, 2,243MiB Writes Queued: 226,242, 904,968KiB
Read Dispatches: 544,701, 2,243MiB Write Dispatches: 159,318, 904,968KiB
Reads Requeued: 0 Writes Requeued: 0
Reads Completed: 544,716, 2,243MiB Writes Completed: 159,321, 904,980KiB
Read Merges: 16,187, 64,748KiB Write Merges: 61,744, 246,976KiB
IO unplugs: 149,614 Timer unplugs: 2,940
With the patch:
Reads Queued: 734,315, 2,937MiB Writes Queued: 300,188, 1,200MiB
Read Dispatches: 214,972, 2,937MiB Write Dispatches: 215,176, 1,200MiB
Reads Requeued: 0 Writes Requeued: 0
Reads Completed: 214,971, 2,937MiB Writes Completed: 215,177, 1,200MiB
Read Merges: 519,343, 2,077MiB Write Merges: 73,325, 293,300KiB
IO unplugs: 337,130 Timer unplugs: 11,184
Signed-off-by: Christian Ehrhardt <ehrhardt@xxxxxxxxxxxxxxxxxx>
Acked-by: Rik van Riel <riel@xxxxxxxxxx>
Acked-by: Jens Axboe <axboe@xxxxxxxxx>
Reviewed-by: Minchan Kim <minchan@xxxxxxxxxx>
---
mm/swap_state.c | 5 +++++
1 files changed, 5 insertions(+), 0 deletions(-)
diff --git a/mm/swap_state.c b/mm/swap_state.c
index 4c5ff7f..c85b559 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -14,6 +14,7 @@
#include <linux/init.h>
#include <linux/pagemap.h>
#include <linux/backing-dev.h>
+#include <linux/blkdev.h>
#include <linux/pagevec.h>
#include <linux/migrate.h>
#include <linux/page_cgroup.h>
@@ -376,6 +377,7 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
unsigned long offset = swp_offset(entry);
unsigned long start_offset, end_offset;
unsigned long mask = (1UL << page_cluster) - 1;
+ struct blk_plug plug;
/* Read a page_cluster sized and aligned cluster around offset. */
start_offset = offset & ~mask;
@@ -383,6 +385,7 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
if (!start_offset) /* First page is swap header. */
start_offset++;
+ blk_start_plug(&plug);
for (offset = start_offset; offset <= end_offset ; offset++) {
/* Ok, do the async read-ahead now */
page = read_swap_cache_async(swp_entry(swp_type(entry), offset),
@@ -391,6 +394,8 @@ struct page *swapin_readahead(swp_entry_t entry, gfp_t gfp_mask,
continue;
page_cache_release(page);
}
+ blk_finish_plug(&plug);
+
lru_add_drain(); /* Push any new pages onto the LRU now */
return read_swap_cache_async(entry, gfp_mask, vma, addr);
}
--
1.7.0.4
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