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SubjectRe: [RFC] (How to) Let idle CPUs sleep
On Thu, Jun 30, 2005 at 06:17:11PM +0530, Srivatsa Vaddagiri wrote:
> Digging further revealed that this max time was restricted by
> various timers kernel uses. Mostly it was found to be because of
> the slab allocator reap timer (it requests a timer every ~2sec on
> every CPU) and machine_check timer (MCE_RATE in arch/i386/kernel/cpu/mcheck/
> non-fatal.c ).

I modified the slab allocator to do a slightly better job of handling timers.
(instead of blindly increasing the reap timeout to 20 sec as I did in my last
run). The patch below is merely a hack meant to see what kind of benefits we
can possibly hope to get by reworking the reap timer. A good solution would
probably increase/decrease the reap periods based on memory pressure and
idleness of the system. Anyway the patch I tried out is:


---

linux-2.6.13-rc1-root/mm/slab.c | 40 ++++++++++++++++++++++++++++++++--------
1 files changed, 32 insertions(+), 8 deletions(-)

diff -puN mm/slab.c~vst-slab mm/slab.c
--- linux-2.6.13-rc1/mm/slab.c~vst-slab 2005-07-05 16:36:03.000000000 +0530
+++ linux-2.6.13-rc1-root/mm/slab.c 2005-07-06 18:01:28.000000000 +0530
@@ -371,6 +371,8 @@ struct kmem_cache_s {
*/
#define REAPTIMEOUT_CPUC (2*HZ)
#define REAPTIMEOUT_LIST3 (4*HZ)
+#define MAX_REAP_TIMEOUT (30*HZ)
+#define MAX_DRAIN_COUNT 2

#if STATS
#define STATS_INC_ACTIVE(x) ((x)->num_active++)
@@ -569,6 +571,7 @@ static enum {
} g_cpucache_up;

static DEFINE_PER_CPU(struct work_struct, reap_work);
+static DEFINE_PER_CPU(unsigned long, last_timeout);

static void free_block(kmem_cache_t* cachep, void** objpp, int len);
static void enable_cpucache (kmem_cache_t *cachep);
@@ -883,8 +886,10 @@ static int __init cpucache_init(void)
* pages to gfp.
*/
for (cpu = 0; cpu < NR_CPUS; cpu++) {
- if (cpu_online(cpu))
+ if (cpu_online(cpu)) {
+ per_cpu(last_timeout, cpu) = REAPTIMEOUT_CPUC + cpu;
start_cpu_timer(cpu);
+ }
}

return 0;
@@ -1543,7 +1548,7 @@ static void smp_call_function_all_cpus(v
preempt_enable();
}

-static void drain_array_locked(kmem_cache_t* cachep,
+static int drain_array_locked(kmem_cache_t* cachep,
struct array_cache *ac, int force);

static void do_drain(void *arg)
@@ -2753,10 +2758,10 @@ static void enable_cpucache(kmem_cache_t
cachep->name, -err);
}

-static void drain_array_locked(kmem_cache_t *cachep,
+static int drain_array_locked(kmem_cache_t *cachep,
struct array_cache *ac, int force)
{
- int tofree;
+ int tofree = 1;

check_spinlock_acquired(cachep);
if (ac->touched && !force) {
@@ -2771,6 +2776,8 @@ static void drain_array_locked(kmem_cach
memmove(&ac_entry(ac)[0], &ac_entry(ac)[tofree],
sizeof(void*)*ac->avail);
}
+
+ return tofree;
}

/**
@@ -2787,17 +2794,20 @@ static void drain_array_locked(kmem_cach
static void cache_reap(void *unused)
{
struct list_head *walk;
+ int drain_count = 0, freed_slab_count = 0;
+ unsigned long timeout = __get_cpu_var(last_timeout);
+ int cpu = smp_processor_id();

if (down_trylock(&cache_chain_sem)) {
/* Give up. Setup the next iteration. */
- schedule_delayed_work(&__get_cpu_var(reap_work), REAPTIMEOUT_CPUC + smp_processor_id());
+ schedule_delayed_work(&__get_cpu_var(reap_work), timeout);
return;
}

list_for_each(walk, &cache_chain) {
kmem_cache_t *searchp;
struct list_head* p;
- int tofree;
+ int tofree, count;
struct slab *slabp;

searchp = list_entry(walk, kmem_cache_t, next);
@@ -2809,7 +2819,9 @@ static void cache_reap(void *unused)

spin_lock_irq(&searchp->spinlock);

- drain_array_locked(searchp, ac_data(searchp), 0);
+ count = drain_array_locked(searchp, ac_data(searchp), 0);
+ if (count > drain_count)
+ drain_count = count;

if(time_after(searchp->lists.next_reap, jiffies))
goto next_unlock;
@@ -2825,6 +2837,9 @@ static void cache_reap(void *unused)
}

tofree = (searchp->free_limit+5*searchp->num-1)/(5*searchp->num);
+ if (tofree > freed_slab_count)
+ freed_slab_count = tofree;
+
do {
p = list3_data(searchp)->slabs_free.next;
if (p == &(list3_data(searchp)->slabs_free))
@@ -2854,7 +2869,16 @@ next:
up(&cache_chain_sem);
drain_remote_pages();
/* Setup the next iteration */
- schedule_delayed_work(&__get_cpu_var(reap_work), REAPTIMEOUT_CPUC + smp_processor_id());
+#ifdef CONFIG_NO_IDLE_HZ
+ if (drain_count < MAX_DRAIN_COUNT && !freed_slab_count) {
+ if (timeout * 2 < MAX_REAP_TIMEOUT)
+ timeout *= 2;
+ } else
+#endif
+ timeout = REAPTIMEOUT_CPUC + cpu;
+
+ __get_cpu_var(last_timeout) = timeout;
+ schedule_delayed_work(&__get_cpu_var(reap_work), timeout);
}

#ifdef CONFIG_PROC_FS
_
The results with this patch on a 8way Intel box are:

CPU# # of Mean Std Dev Max Min
samples


1: 31 4124.742 4331.914 14317.000 0.000
2: 35 3603.600 3792.050 12556.000 14.000
3: 2585 49.458 4.207 50.000 2.000
4: 151 847.682 329.343 1139.000 15.000
5: 23 5432.652 3461.856 12024.000 120.000
6: 19 6229.158 5641.813 15000.000 169.000
7: 67 1865.672 1528.343 5000.000 19.000


Note that the best average is around ~6sec (with max being 15 sec).

Given, this do you still advocate that we restrict idle CPUs to wakeup
every 100ms and check for imbalance? IMO, we should let them sleep
much longer (few seconds) ..What are the consequences on load balancing
if idle CPUs sleep that long? Does it mean that system can remain unresponsive
for few seconds under some circumstances (when there is a burst of activity and
at that time idle CPUs are sleeping)?


--


Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017
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