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Message-ID: <56091651.6070607@odin.com>
Date: Mon, 28 Sep 2015 13:28:33 +0300
From: Kirill Tkhai <ktkhai@...n.com>
To: Mike Galbraith <umgwanakikbuti@...il.com>
CC: <linux-kernel@...r.kernel.org>,
Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] sched/fair: Skip wake_affine() for core siblings
On 26.09.2015 18:25, Mike Galbraith wrote:
> On Fri, 2015-09-25 at 20:54 +0300, Kirill Tkhai wrote:
>> We are not interested in actual target if both prev
>> and curr cpus share CPU cache. select_idle_sibling()
>> searches in top-down order; top level is the same
>> for both of them, and the result will be the same.
>> So, we can save a little CPU cycles and cache misses
>> and skip wake_affine() calculations.
>
> But, whereas previously wake_affine() could NAK a migration if it would
> create an imbalance, we'll now just go ahead and stack tasks if
> select_idle_sibling() can't find an idle home to override the blanket
> approval. It doesn't look like a good idea to me to bounce tasks around
> only to then perhaps stack them, as if we do stack waker/wakee, we
> certainly lose concurrency. (microbenchmarks like pipe-test love that,
> but not all that many real applications play ping-pong for a living;)
>
> I spent most of the day piddling with your little patch, so I'll post
> some condensed mixed load notes.
>
> concurrent tbench 4 + pgbench, 30 seconds per client count (i4790+smt)
> master master+
> pgbench 1 2 3 avg 1 2 3 avg comp
> clients 1 tps = 18768 18591 18264 18541 18351 17257 17245 17617 .950
> clients 2 tps = 30779 30661 31016 30818 29112 28026 29026 28721 .931
> clients 4 tps = 54195 55100 54048 54447 53290 52336 52930 52852 .970
> clients 8 tps = 60332 67052 64699 64027 38491 35746 37746 37327 .582!!
Yeah, this is terrible.
> Do the opposite, wake_affine() always NAKs.
> master master++
> pgbench 1 2 3 avg 1 2 3 avg comp
> clients 1 tps = 18768 18591 18264 18541 16874 16865 16665 16801 .906
> clients 2 tps = 30779 30661 31016 30818 33562 33546 33681 33596 1.090
> clients 4 tps = 54195 55100 54048 54447 61544 61482 61117 61381 1.127
> clients 8 tps = 60332 67052 64699 64027 75171 75524 75318 75337 1.176
Looks like, NAK may be better, because it saves L1 cache, while the patch always invalidates it.
Could you say, do you execute pgbench using just -cX -jY -T30 or something special? I've tried it,
but the dispersion of the results much differs from time to time.
>
> ...
>
> virgin vs your patch again, 2 _minutes_ per client count, as I noticed much variance at 8
> clients, where wake_wide() is supposed to kick in to keep N:M load spread out.
>
> master master+
> pgbench 1 2 3 avg 1 2 3 avg comp
> clients 1 tps = 18548 18673 18390 18537 17879 17652 17621 17717 .955
> clients 2 tps = 31083 31110 30859 31017 30274 30003 29796 30024 .967
> clients 4 tps = 53107 53156 53601 53288 52658 53024 53449 53043 .995
> clients 8 tps = 34213 34310 28844 32455 31360 31416 30732 31169 .960
>
> 30 seconds per run isn't enough, and wake_wide() is not doing a wonderful job for 1:N pgbench.
>
> hrmph, twiddle...
>
> waker/wakee coupling strengthened
> postgres@...er:~> pgbench.sh
> clients 1 tps = 18035
> clients 2 tps = 32525
> clients 4 tps = 53246
> clients 8 tps = 37278
>
> better, but not enough.. + sd_llc_size = #cores vs #threads
> postgres@...er:~> pgbench.sh
> clients 1 tps = 18482
> clients 2 tps = 32366
> clients 4 tps = 54557
> clients 8 tps = 69643
>
> Ok, that's what I want to see, full repeat.
> master = twiddle
> master+ = twiddle+patch
>
> concurrent tbench 4 + pgbench, 2 minutes per client count (i4790+smt)
> master master+
> pgbench 1 2 3 avg 1 2 3 avg comp
> clients 1 tps = 18599 18627 18532 18586 17480 17682 17606 17589 .946
> clients 2 tps = 32344 32313 32408 32355 25167 26140 23730 25012 .773
> clients 4 tps = 52593 51390 51095 51692 22983 23046 22427 22818 .441
> clients 8 tps = 70354 69583 70107 70014 66924 66672 69310 67635 .966
>
> Hrm... turn the tables, measure tbench while pgbench 4 client load runs endlessly.
>
> master master+
> tbench 1 2 3 avg 1 2 3 avg comp
> pairs 1 MB/s = 430 426 436 430 481 481 494 485 1.127
> pairs 2 MB/s = 1083 1085 1072 1080 1086 1090 1083 1086 1.005
> pairs 4 MB/s = 1725 1697 1729 1717 2023 2002 2006 2010 1.170
> pairs 8 MB/s = 2740 2631 2700 2690 3016 2977 3071 3021 1.123
>
> tbench without competition
> master master+ comp
> pairs 1 MB/s = 694 692 .997
> pairs 2 MB/s = 1268 1259 .992
> pairs 4 MB/s = 2210 2165 .979
> pairs 8 MB/s = 3586 3526 .983 (yawn, all within routine variance)
Hm, it seems tbench with competition is better only because of a busy system makes tbench
processes be woken on the same cpu.
> twiddle:
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6048,14 +6048,18 @@ static void update_top_cache_domain(int
> {
> struct sched_domain *sd;
> struct sched_domain *busy_sd = NULL;
> + struct sched_group *group;
> int id = cpu;
> int size = 1;
>
> sd = highest_flag_domain(cpu, SD_SHARE_PKG_RESOURCES);
> if (sd) {
> id = cpumask_first(sched_domain_span(sd));
> - size = cpumask_weight(sched_domain_span(sd));
> busy_sd = sd->parent; /* sd_busy */
> + group = sd->groups;
> + /* Set size to the number of cores, not threads */
> + while (group = group->next, group != sd->groups)
> + size++;
> }
> rcu_assign_pointer(per_cpu(sd_busy, cpu), busy_sd);
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4421,19 +4421,26 @@ static unsigned long cpu_avg_load_per_ta
>
> static void record_wakee(struct task_struct *p)
> {
> + unsigned long now = jiffies;
> +
> /*
> * Rough decay (wiping) for cost saving, don't worry
> * about the boundary, really active task won't care
> * about the loss.
> */
> - if (time_after(jiffies, current->wakee_flip_decay_ts + HZ)) {
> + if (time_after(now, current->wakee_flip_decay_ts + HZ)) {
> current->wakee_flips >>= 1;
> - current->wakee_flip_decay_ts = jiffies;
> + current->wakee_flip_decay_ts = now;
> + }
> + if (time_after(now, p->wakee_flip_decay_ts + HZ)) {
> + p->wakee_flips >>= 1;
> + p->wakee_flip_decay_ts = now;
> }
>
> if (current->last_wakee != p) {
> current->last_wakee = p;
> current->wakee_flips++;
> + p->wakee_flips++;
> }
> }
>
>
Regards,
Kirill
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