On Wed, 28 Apr 2010 10:04:32 -0500
Jack Steiner <steiner@xxxxxxx> wrote:
> Some workloads that create a large number of small files tend to assign
> too many pages to node 0 (multi-node systems). Part of the reason is that
> the rotor (in cpuset_mem_spread_node()) used to assign nodes starts
> at node 0 for newly created tasks.
And, presumably, your secret testcase forks lots of subprocesses which
do the file creation?
> This patch changes the rotor to be initialized to a random node number
> of the cpuset.
Why random as opposed to, say, inherit-rotor-from-parent?
> Index: linux/arch/x86/mm/numa.c
> ===================================================================
> --- linux.orig/arch/x86/mm/numa.c 2010-04-28 09:44:52.422898844 -0500
> +++ linux/arch/x86/mm/numa.c 2010-04-28 09:49:39.282899779 -0500
> @@ -2,6 +2,7 @@
> #include <linux/topology.h>
> #include <linux/module.h>
> #include <linux/bootmem.h>
> +#include <linux/random.h>
>
> #ifdef CONFIG_DEBUG_PER_CPU_MAPS
> # define DBG(x...) printk(KERN_DEBUG x)
> @@ -65,3 +66,19 @@ const struct cpumask *cpumask_of_node(in
> }
> EXPORT_SYMBOL(cpumask_of_node);
> #endif
> +
> +/*
> + * Return the bit number of a random bit set in the nodemask.
> + * (returns -1 if nodemask is empty)
> + */
> +int __node_random(const nodemask_t *maskp)
> +{
> + int w, bit = -1;
> +
> + w = nodes_weight(*maskp);
> + if (w)
> + bit = bitmap_find_nth_bit(maskp->bits,
> + get_random_int() % w, MAX_NUMNODES);
> + return bit;
> +}
> +EXPORT_SYMBOL(__node_random);
I suspect random32() would suffice here. It avoids depleting the
entropy pool altogether.
> +
> +/**
> + * bitmap_find_nth_bit(buf, ord, bits)
> + * @buf: pointer to bitmap
> + * @n: ordinal bit position (n-th set bit, n >= 0)
> + * @nbits: number of bits in the bitmap
> + *
> + * find the Nth bit that is set in the bitmap
> + * Value of @n should be in range 0 <= @n < weight(buf), else
> + * results are undefined.
> + *
> + * The bit positions 0 through @bits are valid positions in @buf.
> + */
> +int bitmap_find_nth_bit(const unsigned long *bitmap, int n, int bits)
> +{
> + return bitmap_ord_to_pos(bitmap, n, bits);
> +}
> +EXPORT_SYMBOL(bitmap_find_nth_bit);
This does nothing apart from consume more stack? Better to rename
bitmap_ord_to_pos() and export it.
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