[Patch] Scale pidhash_shift/pidhash_size up based onnum_possible_cpus().

From: Robin Holt
Date: Thu Jul 31 2008 - 13:00:45 EST



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@xxxxxxx>


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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