linux-kernel - [PATCH 2/2] fdtable: Avoid fdset cacheline ping-pong.

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Message-Id: <200610141629.19522.vlobanov@speakeasy.net>
Date:	Sat, 14 Oct 2006 16:29:19 -0700
From:	Vadim Lobanov <vlobanov@...akeasy.net>
To:	akpm@...l.org
Cc:	linux-kernel@...r.kernel.org
Subject: [PATCH 2/2] fdtable: Avoid fdset cacheline ping-pong.

This is a performance optimization for the fdtable code. When expanding the
fdtable for a task, we want to allocate at least L1_CACHE_BYTES for the new
fdset memory -- this will ensure that the fdsets of two different tasks will
not share the same cacheline, causing lots of cacheline ping-pongs.

Signed-off-by: Vadim Lobanov <vlobanov@...akeasy.net>

diff -Npru old/fs/file.c new/fs/file.c
--- old/fs/file.c	2006-10-14 15:40:12.000000000 -0700
+++ new/fs/file.c	2006-10-14 15:50:00.000000000 -0700
@@ -178,7 +178,8 @@ static struct fdtable * alloc_fdtable(un
 	if (!data)
 		goto out_fdt;
 	fdt->fd = (struct file **)data;
-	data = alloc_fdmem(2 * nr / BITS_PER_BYTE);
+	data = alloc_fdmem(max_t(unsigned int,
+				 2 * nr / BITS_PER_BYTE, L1_CACHE_BYTES));
 	if (!data)
 		goto out_arr;
 	fdt->open_fds = (fd_set *)data;
-
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