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fs/buffer.c: make bh_lru_install() more efficient

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To install a buffer_head into the cpu's LRU queue, bh_lru_install()
would construct a new copy of the queue and then memcpy it over the real
queue.  But it's easily possible to do the update in-place, which is
faster and simpler.  Some work can also be skipped if the buffer_head
was already in the queue.

As a microbenchmark I timed how long it takes to run sb_getblk()
10,000,000 times alternating between BH_LRU_SIZE + 1 blocks.
Effectively, this benchmarks looking up buffer_heads that are in the
page cache but not in the LRU:

	Before this patch: 1.758s
	After this patch: 1.653s

This patch also removes about 350 bytes of compiled code (on x86_64),
partly due to removal of the memcpy() which was being inlined+unrolled.

Link: http://lkml.kernel.org/r/20161229193445.1913-1-ebiggers3@gmail.com
Signed-off-by: Eric Biggers <ebiggers@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Lameter <cl@linux.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Eric Biggers 2017-07-10 15:47:29 -07:00 committed by Linus Torvalds
parent 3457f41476
commit 241f01fbed
1 changed files with 15 additions and 28 deletions
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43
fs/buffer.c
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@ -1281,44 +1281,31 @@ static inline void check_irqs_on(void)
}
/*
* The LRU management algorithm is dopey-but-simple. Sorry.
* Install a buffer_head into this cpu's LRU. If not already in the LRU, it is
* inserted at the front, and the buffer_head at the back if any is evicted.
* Or, if already in the LRU it is moved to the front.
*/
static void bh_lru_install(struct buffer_head *bh)
{
struct buffer_head *evictee = NULL;
struct buffer_head *evictee = bh;
struct bh_lru *b;
int i;
check_irqs_on();
bh_lru_lock();
if (__this_cpu_read(bh_lrus.bhs[0]) != bh) {
struct buffer_head *bhs[BH_LRU_SIZE];
int in;
int out = 0;
get_bh(bh);
bhs[out++] = bh;
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) {
BUG_ON(evictee != NULL);
evictee = bh2;
} else {
bhs[out++] = bh2;
}
}
b = this_cpu_ptr(&bh_lrus);
for (i = 0; i < BH_LRU_SIZE; i++) {
swap(evictee, b->bhs[i]);
if (evictee == bh) {
bh_lru_unlock();
return;
}
while (out < BH_LRU_SIZE)
bhs[out++] = NULL;
memcpy(this_cpu_ptr(&bh_lrus.bhs), bhs, sizeof(bhs));
}
bh_lru_unlock();
if (evictee)
__brelse(evictee);
get_bh(bh);
bh_lru_unlock();
brelse(evictee);
}
/*