runtime: frequent blocking syscalls cause high sysmon overhead

[philhofer]

The hard-coded 20us sysmon tick in the runtime appears to be far too aggressive for minority architectures.

Our storage controllers run a single-core 800MHz arm cpu, which runs single-threaded workloads between 10 and 20 times slower than my desktop Intel machine.

Most of our product-level benchmarks improved substantially (5-10%) when I manually adjusted the minimum tick to 2ms on our arm devices. (Our use-case benefits from particularly long syscall ticks, since we tend to make read and write syscalls with large buffers, and those can take a long time even if they never actually block in I/O. Also, as you might imagine, the relative cost of thread creation and destruction on this system is quite high.)

Perhaps the runtime should calibrate the sysmon tick based on the performance of a handful of trivial syscalls (getpid(), etc). Similarly, the tick could be adjusted upwards if an m wakes up within a millisecond (or so) of being kicked off its p. Perhaps someone with a better intuition for the runtime can suggest something heuristically less messy.

Thanks,
Phil

[bradfitz]

/cc @minux @cherrymui @davecheney

[davecheney]

That looks like a marvel ARMv5 chip, which we're proposing on dropping in Go 1.9

…

On Thu, Dec 8, 2016 at 6:49 AM, Brad Fitzpatrick ***@***.***> wrote: /cc @minux <https://github.com/minux> @cherrymui <https://github.com/cherrymui> @davecheney <https://github.com/davecheney> — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#18236 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAAcA_qAG040kCGVl0TpdSTSaNv1sYOAks5rFyn5gaJpZM4LHJlV> .

[philhofer]

It's a Marvell Armada 370, which is ARMv7

[davecheney]

Phew!

…

On Thu, Dec 8, 2016 at 7:34 AM, Philip Hofer ***@***.***> wrote: It's a Marvell Armada 370, which is ARMv7 — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#18236 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAAcA280K1VdUNc9HYwq-_oJta9rUPfWks5rFzRkgaJpZM4LHJlV> .

[philhofer]

Yes, please drop ARMv5. I was secretly elated when I saw that was finally happening. There are too many wildly divergent implementations of ARM in the wild.

[minux]

Yeah, I think timing a few getpid syscalls and adjust sysmon tick accordingly is reasonable.

[aclements]

In general, sysmon should back off to a 10ms delay unless it's frequently finding goroutines that are blocked in syscalls for more than 10ms or there are frequent periods where all threads are idle (e.g., waiting on the network or blocked in syscalls). Is that the case in your application?

Perhaps the runtime should calibrate the sysmon tick based on the performance of a handful of trivial syscalls (getpid(), etc). Similarly, the tick could be adjusted upwards if an m wakes up within a millisecond (or so) of being kicked off its p.

Perhaps I don't understand your application, but I don't see how these would help. There are two delays involved here: one is the minimum sysmon delay, which is 20us, and the other is the retake delay, which is 10ms. I think both of your suggestions aim at adjusting the retake delay, but it sounds like that's not the problem here.

I think the best way to handle the minimum delay is to eliminate it by making sysmon event-driven. The minimum delay is there because sysmon is sampling what is ultimately a stream of instantaneous events. I believe it could instead compute when the next interesting event will occur and just sleep for that long. This is slightly complicated by how we figure out how long a goroutine has been executing or in a syscall, but in this model the minimum delay controls the variance we're willing to accept on that. In practice, that variance can be up to 10ms right now, which would suggest a minimum tick of 10ms if sysmon were event-driven.

[philhofer]

@aclements Thanks for the detailed reply.

We do, in fact, have all threads blocked in syscalls quite frequently. There's only one P, and the only meaningful thing that this hardware does is move data from the network to the disk and vice-versa. (We spend a good chunk of time in startm and stopm.) Many of these syscalls don't truly "block," because we do a relatively good job of issuing the right prefetch hints to hit the kernel block cache, but inevitably blocking requests start to pile up.

Sorta related: #6817, #7903 (we do already have a sempahore around blocking syscalls)

Probably also worth pointing out that this hardware definitely can't hit a 1ms timer deadline reliably under linux, and struggles to meet a 10ms deadline with less than 2ms of overrun.

[philhofer]

In case it helps, here's the hack that bought us a perf boost:

diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index cad1b1c..5fc3dcd 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -3753,7 +3753,11 @@ func sysmon() {
        delay := uint32(0)
        for {
                if idle == 0 { // start with 20us sleep...
-                       delay = 20
+                       if GOARCH == "arm" {
+                               delay = 2 * 1000
+                       } else {
+                               delay = 20
+                       }
                } else if idle > 50 { // start doubling the sleep after 1ms...
                        delay *= 2
                }

[bradfitz]

Sorry, bumping this again. (But I assume Igneous has been running their own builds for the past two years anyway)

[jeffdh]

Thanks for the bump @bradfitz. We have opted to put a semaphore in front of disk I/O that keeps things in optimal syscall/thread territory to avoid this issue as opposed to the patch listed above.