linux-kernel - [Patch] Scale pidhash_shift/pidhash_size up based on num_possible

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Date:	Thu, 31 Jul 2008 12:00:22 -0500
From:	Robin Holt <holt@....com>
To:	linux-kernel@...r.kernel.org
Cc:	Pavel Emelyanov <xemul@...nvz.org>,
	Oleg Nesterov <oleg@...sign.ru>,
	Sukadev Bhattiprolu <sukadev@...ibm.com>,
	Paul Menage <menage@...gle.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [Patch] Scale pidhash_shift/pidhash_size up based on
	num_possible_cpus().


For large cpu configurations, we find the number of pids in a pidhash
bucket cause things like 'ps' to perform slowly.  Raising pidhash_shift
from 12 to 16 cut the time for 'ps' in half on a 2048 cpu machine.

This patch makes the upper limit scale based upon num_possible_cpus().
For machines 128 cpus or less, the current upper limit of 12 is
maintained.

Signed-off-by: Robin Holt <holt@....com>


Index: contention_unroll/kernel/pid.c
===================================================================
--- contention_unroll.orig/kernel/pid.c	2008-07-31 11:59:21.154284073 -0500
+++ contention_unroll/kernel/pid.c	2008-07-31 11:59:22.862497720 -0500
@@ -502,9 +502,10 @@ void __init pidhash_init(void)
 {
 	int i, pidhash_size;
 	unsigned long megabytes = nr_kernel_pages >> (20 - PAGE_SHIFT);
+	int pidhash_shift_ul = max(12, fls(num_possible_cpus() - 1) + 5);
 
 	pidhash_shift = max(4, fls(megabytes * 4));
-	pidhash_shift = min(12, pidhash_shift);
+	pidhash_shift = min(pidhash_shift_ul, pidhash_shift);
 	pidhash_size = 1 << pidhash_shift;
 
 	printk("PID hash table entries: %d (order: %d, %Zd bytes)\n",
--
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