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

[Sebastien Buisson]

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)

The buffer-head per-CPU LRU size can be changed at config time, and its
default value is raised to 16.

Signed-off-by: Liang Zhen <liang.zhen@xxxxxxxxx>
Signed-off-by: Andreas Dilger <andreas.dilger@xxxxxxxxx>
---
 fs/Kconfig  |    9 +++++++++
 fs/buffer.c |    3 +--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/fs/Kconfig b/fs/Kconfig
index c229f82..c08844c 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -268,4 +268,13 @@ endif # NETWORK_FILESYSTEMS
 source "fs/nls/Kconfig"
 source "fs/dlm/Kconfig"

+config BH_LRU_SIZE
+      int
+      range 8 64
+      default "16"
+      help
+        This sets the per-CPU LRU size for buffer heads in memory.
+        More complex filesystems may be modifying multiple blocks
+        within a single transaction, so keeping the buffer heads in
+        CPU-local cache speeds up modifications significantly.
 endmenu
diff --git a/fs/buffer.c b/fs/buffer.c
index 6024877..b83fa63 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1255,8 +1255,7 @@ static struct buffer_head *__bread_slow(struct 
buffer_head *bh)
  * The LRUs themselves only need locking against invalidate_bh_lrus. 
We use
  * a local interrupt disable for that.
  */
-
-#define BH_LRU_SIZE	8
+#define BH_LRU_SIZE	CONFIG_BH_LRU_SIZE

 struct bh_lru {
 	struct buffer_head *bhs[BH_LRU_SIZE];
--
1.7.1



Le 06/07/2014 18:18, Andi Kleen a écrit :
> Sebastien Buisson <sebastien.buisson@xxxxxxxx> writes:
>
> > 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:
>
> I don't think that should be a user visible config. Most users wouldn't
> know how to set it. And Linux already has far too many obscure
> config options.
>
> Either make it set implicitely by the file system config.
> Or just increase it unconditionally?
>
> -Andi
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