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Date: Sun, 29 Mar 2009 22:43:39 -0700 (PDT)
From: David Rientjes <rientjes@...gle.com>
To: Pekka Enberg <penberg@...helsinki.fi>
cc: Christoph Lameter <cl@...ux-foundation.org>,
Nick Piggin <nickpiggin@...oo.com.au>,
Martin Bligh <mbligh@...gle.com>, linux-kernel@...r.kernel.org
Subject: [patch 1/3] slub: add per-cache slab thrash ratio
Adds /sys/kernel/slab/cache/slab_thrash_ratio, which represents the
percentage of a slab's objects that the fastpath must fulfill to not be
considered thrashing on a per-cpu basis[*].
"Thrashing" here is defined as the constant swapping of the cpu slab such
that the slowpath is followed the majority of the time because the
refilled cpu slab can only accommodate a small number of allocations.
This occurs when the object allocation and freeing pattern for a cache is
such that it spends more time swapping the cpu slab than fulfulling
fastpath allocations.
[*] A single instance of the thrash ratio not being reached in the
fastpath does not indicate the cpu cache is thrashing. A
pre-defined value will later be added to determine how many times
the ratio must not be reached before a cache is actually thrashing.
This is defined as a ratio based on the number of objects in a cache's
slab. This is automatically changed when /sys/kernel/slab/cache/order is
changed to reflect the same ratio.
The netperf TCP_RR benchmark illustrates slab thrashing very well with a
large number of threads. With a test length of 60 seconds, the following
thread counts were used to show the effect of the allocation and freeing
pattern of such a workload.
Before this patchset:
threads Transfer Rate (per sec)
16 71592
32 95373
48 113072
64 149043
80 172035
96 187849
112 204962
128 217547
144 232369
160 239871
176 242712
192 243182
To identify the thrashing caches, the same workload was run with
CONFIG_SLUB_STATS enabled. The following caches are obviously performing
very poorly:
cache ALLOC_FASTPATH ALLOC_SLOWPATH
kmalloc-256 98125871 31585955
kmalloc-2048 77243698 52347453
cache FREE_FASTPATH FREE_SLOWPATH
kmalloc-256 173624 129538000
kmalloc-2048 90520 129500630
After this patchset (both caches with slab_thrash_ratios of 20):
threads Transfer Rate (per sec)
16 69505
32 119731
48 125014
64 158919
80 179679
96 192154
112 209988
128 223507
144 234565
160 244789
176 248971
192 255596
Although slabs may accommodate fewer objects than others when contiguous
memory cannot be allocated for a cache's order, the ratio is still based
on its configured `order' since slabs will exist on the partial list that
will be able to fulfill such a requirement.
The value is stored in terms of the number of objects that the ratio
represents, not the ratio itself. This avoids costly arithmetic in the
slowpath for a calculation that could otherwise be done only when
`slab_thrash_ratio' or `order' is changed.
This also will adjust the configured ratio to one that can actually be
represented in terms of whole numbers: for example, if slab_thrash_ratio
is set to 20 for a cache with 64 objects, the effective ratio is actually
3:16 (or 18.75%). This will be shown when reading the ratio since it is
better to represent the actual ratio instead of a pseudo substitute.
The slab_thrash_ratio for each cache do not have non-zero defaults
(yet?).
Cc: Christoph Lameter <cl@...ux-foundation.org>
Cc: Nick Piggin <nickpiggin@...oo.com.au>
Signed-off-by: David Rientjes <rientjes@...gle.com>
---
include/linux/slub_def.h | 1 +
mm/slub.c | 29 +++++++++++++++++++++++++++++
2 files changed, 30 insertions(+), 0 deletions(-)
diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
--- a/include/linux/slub_def.h
+++ b/include/linux/slub_def.h
@@ -94,6 +94,7 @@ struct kmem_cache {
#ifdef CONFIG_SLUB_DEBUG
struct kobject kobj; /* For sysfs */
#endif
+ u16 min_free_watermark; /* Calculated from slab thrash ratio */
#ifdef CONFIG_NUMA
/*
diff --git a/mm/slub.c b/mm/slub.c
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -2186,6 +2186,7 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
unsigned long flags = s->flags;
unsigned long size = s->objsize;
unsigned long align = s->align;
+ u16 thrash_ratio = 0;
int order;
/*
@@ -2291,10 +2292,13 @@ static int calculate_sizes(struct kmem_cache *s, int forced_order)
/*
* Determine the number of objects per slab
*/
+ if (oo_objects(s->oo))
+ thrash_ratio = s->min_free_watermark * 100 / oo_objects(s->oo);
s->oo = oo_make(order, size);
s->min = oo_make(get_order(size), size);
if (oo_objects(s->oo) > oo_objects(s->max))
s->max = s->oo;
+ s->min_free_watermark = oo_objects(s->oo) * thrash_ratio / 100;
return !!oo_objects(s->oo);
@@ -2321,6 +2325,7 @@ static int kmem_cache_open(struct kmem_cache *s, gfp_t gfpflags,
*/
set_min_partial(s, ilog2(s->size));
s->refcount = 1;
+ s->min_free_watermark = 0;
#ifdef CONFIG_NUMA
s->remote_node_defrag_ratio = 1000;
#endif
@@ -4110,6 +4115,29 @@ static ssize_t remote_node_defrag_ratio_store(struct kmem_cache *s,
SLAB_ATTR(remote_node_defrag_ratio);
#endif
+static ssize_t slab_thrash_ratio_show(struct kmem_cache *s, char *buf)
+{
+ return sprintf(buf, "%d\n",
+ s->min_free_watermark * 100 / oo_objects(s->oo));
+}
+
+static ssize_t slab_thrash_ratio_store(struct kmem_cache *s, const char *buf,
+ size_t length)
+{
+ unsigned long ratio;
+ int err;
+
+ err = strict_strtoul(buf, 10, &ratio);
+ if (err)
+ return err;
+
+ if (ratio <= 100)
+ s->min_free_watermark = oo_objects(s->oo) * ratio / 100;
+
+ return length;
+}
+SLAB_ATTR(slab_thrash_ratio);
+
#ifdef CONFIG_SLUB_STATS
static int show_stat(struct kmem_cache *s, char *buf, enum stat_item si)
{
@@ -4194,6 +4222,7 @@ static struct attribute *slab_attrs[] = {
&shrink_attr.attr,
&alloc_calls_attr.attr,
&free_calls_attr.attr,
+ &slab_thrash_ratio_attr.attr,
#ifdef CONFIG_ZONE_DMA
&cache_dma_attr.attr,
#endif
--
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