[PATCH] Allow increasing the buffer-head per-CPU LRU size

From: Sebastien Buisson
Date: Tue Jun 24 2014 - 12:20:37 EST

Influence of buffer-head per-CPU LRU size on metadata performance has been studied with mdtest, on one ext4 formatted ramdisk device, creating, stating and removing 1000000 files in the same directory. Several test cases were evaluated, varying the 'size' of the directory in which files are created:
- target directory is empty
- target directory already contains 100000 files
- target directory already contains 500000 files
- target directory already contains 2000000 files
- target directory already contains 5000000 files
- target directory already contains 10000000 files

To compare the effect of the patch, the same series of tests was run with:
- a vanilla kernel
- a patched kernel with BH_LRU_SIZE set to 16

The tests launched were:
(a) mdtest on ramdisk device, single shared dir, with large ACL and SELinux
(b) mdtest on ramdisk device, single shared dir, with large ACL but NO SELinux

Below are the results showing performance gain (in percentage) when increasing BH_LRU_SIZE to 16 (vanilla default value is 8):
(a)
files tasks dir size Creation Stat Removal
1000000 1 0 -8,7 -2,7 -0,5
1000000 1 100000 -5,2 -0,5 -1,1
1000000 1 500000 -5,1 -3,7 -1,5
1000000 1 2000000 -5,1 -4,0 -8,5
1000000 1 5000000 -4,2 -5,3 -10,2
1000000 1 10000000 -3,5 -8,0 -10,9
1000000 8 0 -0,3 -3,8 -1,2
1000000 8 100000 -1,2 -3,7 -1,5
1000000 8 500000 0,5 -3,2 -5,3
1000000 8 2000000 -1,7 -6,1 -8,7
1000000 8 5000000 -5,9 -7,7 -11,9
1000000 8 10000000 -4,1 -8,8 -13,6

(b)
files tasks dir size Creation Stat Removal
1000000 1 0 0,0 -0,9 -1,1
1000000 1 100000 1,0 -3,0 -3,5
1000000 1 500000 3,7 -3,0 -2,4
1000000 1 2000000 1,1 3,6 -0,2
1000000 1 5000000 3,5 0,1 5,9
1000000 1 10000000 9,0 3,8 6,4
1000000 8 0 2,4 -1,2 -4,3
1000000 8 100000 -0,2 -1,8 -2,4
1000000 8 500000 1,1 -0,3 2,0
1000000 8 2000000 -0,3 -2,8 -3,3
1000000 8 5000000 0,3 -3,1 -1,3
1000000 8 10000000 1,5 0,0 0,7


To sum up briefly, it is very difficult to show performance improvement with mdtest. The only positive case is on Create without SELinux when using 1 thread. Strangely the more threads we have, the poorer is the gain in performance.


Furthermore, metadata tests were run on Lustre with a specific benchmark called mds-survey. They used a ramdisk device, creating, stating and removing 1000000 files.

The tests launched were:
(c) mds-survey on ramdisk device, quota enabled, shared directory
(d) mds-survey on ramdisk device, quota enabled, directory per process

Below are the results showing performance gain (in percentage) when increasing BH_LRU_SIZE to 16 (vanilla default value is 8):
(c)
files dir threads create lookup destroy
1000000 1 1 11,3 1,2 7,2
1000000 1 2 6,4 2,3 6,9
1000000 1 4 1,9 3,0 1,3
1000000 1 8 -0,6 4,3 0,7
1000000 1 16 0,5 4,4 0,6

(d)
files dir threads create lookup destroy
1000000 4 4 3,2 28,5 5,3
1000000 8 8 1,2 33,9 2,0
1000000 16 16 0,6 7,9 -0,2


Compared to pure ext4 tests, we can see more improvements thanks to mds-survey. In shared directory case, gain is between 0 and 10% for create, between 1 and 4% for lookup, and between 0 and 7% for destroy, depending on the number of threads.

All this test plan has been elaborated in collaboration with Intel, and results have been already shared with them.



[PATCH] Allow increasing the buffer-head per-CPU LRU size

Allow increasing the buffer-head per-CPU LRU size to allow efficient
filesystem operations that access many blocks for each transaction.
For example, creating a file in a large ext4 directory with quota
enabled will accesses multiple buffer heads and will overflow the LRU
at the default 8-block LRU size:

* parent directory inode table block (ctime, nlinks for subdirs)
* new inode bitmap
* inode table block
* 2 quota blocks
* directory leaf block (not reused, but pollutes one cache entry)
* 2 levels htree blocks (only one is reused, other pollutes cache)
* 2 levels indirect/index blocks (only one is reused)

Make this tuning be a kernel parameter 'bh_lru_size'.

Signed-off-by: Liang Zhen <liang.zhen@xxxxxxxxx>
Signed-off-by: Andreas Dilger <andreas.dilger@xxxxxxxxx>
Signed-off-by: Sebastien Buisson <sebastien.buisson@xxxxxxxx>
---
Documentation/kernel-parameters.txt | 3 +++
fs/buffer.c | 35 +++++++++++++++++++++++++----------
2 files changed, 28 insertions(+), 10 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 9ca3e74..f0b5b2f 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -480,6 +480,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
Format: <io>,<irq>,<mode>
See header of drivers/net/hamradio/baycom_ser_hdx.c.

+ bh_lru_size= [KNL]
+ Set the buffer-head per-CPU LRU size.
+
blkdevparts= Manual partition parsing of block device(s) for
embedded devices based on command line input.
See Documentation/block/cmdline-partition.txt
diff --git a/fs/buffer.c b/fs/buffer.c
index 6024877..8e987d6 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1256,10 +1256,25 @@ static struct buffer_head *__bread_slow(struct buffer_head *bh)
* a local interrupt disable for that.
*/

-#define BH_LRU_SIZE 8
+#define BH_LRU_SIZE_MAX 64
+
+static unsigned long bh_lru_size = 16;
+static int __init set_bh_lru_size(char *str)
+{
+ if (!str)
+ return 0;
+
+ if (kstrtoul(str, 0, &bh_lru_size))
+ return 0;
+ if (bh_lru_size > BH_LRU_SIZE_MAX)
+ bh_lru_size = BH_LRU_SIZE_MAX;
+
+ return 1;
+}
+__setup("bh_lru_size=", set_bh_lru_size);

struct bh_lru {
- struct buffer_head *bhs[BH_LRU_SIZE];
+ struct buffer_head *bhs[BH_LRU_SIZE_MAX];
};

static DEFINE_PER_CPU(struct bh_lru, bh_lrus) = {{ NULL }};
@@ -1289,20 +1304,20 @@ static void bh_lru_install(struct buffer_head *bh)
check_irqs_on();
bh_lru_lock();
if (__this_cpu_read(bh_lrus.bhs[0]) != bh) {
- struct buffer_head *bhs[BH_LRU_SIZE];
+ struct buffer_head *bhs[BH_LRU_SIZE_MAX];
int in;
int out = 0;

get_bh(bh);
bhs[out++] = bh;
- for (in = 0; in < BH_LRU_SIZE; in++) {
+ for (in = 0; in < bh_lru_size; in++) {
struct buffer_head *bh2 =
__this_cpu_read(bh_lrus.bhs[in]);

if (bh2 == bh) {
__brelse(bh2);
} else {
- if (out >= BH_LRU_SIZE) {
+ if (out >= bh_lru_size) {
BUG_ON(evictee != NULL);
evictee = bh2;
} else {
@@ -1310,7 +1325,7 @@ static void bh_lru_install(struct buffer_head *bh)
}
}
}
- while (out < BH_LRU_SIZE)
+ while (out < BH_LRU_SIZE_MAX)
bhs[out++] = NULL;
memcpy(__this_cpu_ptr(&bh_lrus.bhs), bhs, sizeof(bhs));
}
@@ -1331,7 +1346,7 @@ lookup_bh_lru(struct block_device *bdev, sector_t block, unsigned size)

check_irqs_on();
bh_lru_lock();
- for (i = 0; i < BH_LRU_SIZE; i++) {
+ for (i = 0; i < bh_lru_size; i++) {
struct buffer_head *bh = __this_cpu_read(bh_lrus.bhs[i]);

if (bh && bh->b_bdev == bdev &&
@@ -1437,7 +1452,7 @@ static void invalidate_bh_lru(void *arg)
struct bh_lru *b = &get_cpu_var(bh_lrus);
int i;

- for (i = 0; i < BH_LRU_SIZE; i++) {
+ for (i = 0; i < BH_LRU_SIZE_MAX; i++) {
brelse(b->bhs[i]);
b->bhs[i] = NULL;
}
@@ -1449,7 +1464,7 @@ static bool has_bh_in_lru(int cpu, void *dummy)
struct bh_lru *b = per_cpu_ptr(&bh_lrus, cpu);
int i;

- for (i = 0; i < BH_LRU_SIZE; i++) {
+ for (i = 0; i < bh_lru_size; i++) {
if (b->bhs[i])
return 1;
}
@@ -3359,7 +3374,7 @@ static void buffer_exit_cpu(int cpu)
int i;
struct bh_lru *b = &per_cpu(bh_lrus, cpu);

- for (i = 0; i < BH_LRU_SIZE; i++) {
+ for (i = 0; i < BH_LRU_SIZE_MAX; i++) {
brelse(b->bhs[i]);
b->bhs[i] = NULL;
}
--
1.7.1

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