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https://issues.apache.org/jira/browse/CASSANDRA-13651?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16587624#comment-16587624
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Benedict commented on CASSANDRA-13651:
--------------------------------------
So, I assume we're now defaulting to epoll in most cases, and this behaviour
comes from a period where this wasn't the default (and was probably poorly
justified at the time - AFAICR we used to benchmark with only a single
connection, where this behaviour would be more beneficial).
It's a shame we no longer have any standard benchmarking tools for the project,
but it seems we have multiple data points demonstrating a win (or no loss), and
the code is simpler after the patch.
So, I'm +1 on the patch. I will get a circleci run going shortly.
> Large amount of CPU used by epoll_wait(.., .., .., 0)
> -----------------------------------------------------
>
> Key: CASSANDRA-13651
> URL: https://issues.apache.org/jira/browse/CASSANDRA-13651
> Project: Cassandra
> Issue Type: Bug
> Reporter: Corentin Chary
> Assignee: Corentin Chary
> Priority: Major
> Fix For: 4.x
>
> Attachments: cpu-usage.png
>
>
> I was trying to profile Cassandra under my workload and I kept seeing this
> backtrace:
> {code}
> epollEventLoopGroup-2-3 State: RUNNABLE CPU usage on sample: 240ms
> io.netty.channel.epoll.Native.epollWait0(int, long, int, int) Native.java
> (native)
> io.netty.channel.epoll.Native.epollWait(int, EpollEventArray, int)
> Native.java:111
> io.netty.channel.epoll.EpollEventLoop.epollWait(boolean)
> EpollEventLoop.java:230
> io.netty.channel.epoll.EpollEventLoop.run() EpollEventLoop.java:254
> io.netty.util.concurrent.SingleThreadEventExecutor$5.run()
> SingleThreadEventExecutor.java:858
> io.netty.util.concurrent.DefaultThreadFactory$DefaultRunnableDecorator.run()
> DefaultThreadFactory.java:138
> java.lang.Thread.run() Thread.java:745
> {code}
> At fist I though that the profiler might not be able to profile native code
> properly, but I wen't further and I realized that most of the CPU was used by
> {{epoll_wait()}} calls with a timeout of zero.
> Here is the output of perf on this system, which confirms that most of the
> overhead was with timeout == 0.
> {code}
> Samples: 11M of event 'syscalls:sys_enter_epoll_wait', Event count (approx.):
> 11594448
> Overhead Trace output
>
> ◆
> 90.06% epfd: 0x00000047, events: 0x7f5588c0c000, maxevents: 0x00002000,
> timeout: 0x00000000
> ▒
> 5.77% epfd: 0x000000b5, events: 0x7fca419ef000, maxevents: 0x00001000,
> timeout: 0x00000000
> ▒
> 1.98% epfd: 0x000000b5, events: 0x7fca419ef000, maxevents: 0x00001000,
> timeout: 0x000003e8
> ▒
> 0.04% epfd: 0x00000003, events: 0x2f6af77b9c00, maxevents: 0x00000020,
> timeout: 0x00000000
> ▒
> 0.04% epfd: 0x0000002b, events: 0x121ebf63ac00, maxevents: 0x00000040,
> timeout: 0x00000000
> ▒
> 0.03% epfd: 0x00000026, events: 0x7f51f80019c0, maxevents: 0x00000020,
> timeout: 0x00000000
> ▒
> 0.02% epfd: 0x00000003, events: 0x7fe4d80019d0, maxevents: 0x00000020,
> timeout: 0x00000000
> {code}
> Running this time with perf record -ag for call traces:
> {code}
> # Children Self sys usr Trace output
>
> # ........ ........ ........ ........
> ....................................................................................
> #
> 8.61% 8.61% 0.00% 8.61% epfd: 0x000000a7, events:
> 0x7fca452d6000, maxevents: 0x00001000, timeout: 0x00000000
> |
> ---0x1000200af313
> |
> --8.61%--0x7fca6117bdac
> 0x7fca60459804
> epoll_wait
> 2.98% 2.98% 0.00% 2.98% epfd: 0x000000a7, events:
> 0x7fca452d6000, maxevents: 0x00001000, timeout: 0x000003e8
> |
> ---0x1000200af313
> 0x7fca6117b830
> 0x7fca60459804
> epoll_wait
> {code}
> That looks like a lot of CPU used to wait for nothing. I'm not sure if pref
> reports a per-CPU percentage or a per-system percentage, but that would be
> still be 10% of the total CPU usage of Cassandra at the minimum.
> I went further and found the code of all that: We schedule a lot of
> {{Message::Flusher}} with a deadline of 10 usec (5 per messages I think) but
> netty+epoll only support timeouts above the milliseconds and will convert
> everything bellow to 0.
> I added some traces to netty (4.1):
> {code}
> diff --git
> a/transport-native-epoll/src/main/java/io/netty/channel/epoll/EpollEventLoop.java
>
> b/transport-native-epoll/src/main/java/io/netty/channel/epoll/EpollEventLoop.java
> index 909088fde..8734bbfd4 100644
> ---
> a/transport-native-epoll/src/main/java/io/netty/channel/epoll/EpollEventLoop.java
> +++
> b/transport-native-epoll/src/main/java/io/netty/channel/epoll/EpollEventLoop.java
> @@ -208,10 +208,15 @@ final class EpollEventLoop extends
> SingleThreadEventLoop {
> long currentTimeNanos = System.nanoTime();
> long selectDeadLineNanos = currentTimeNanos +
> delayNanos(currentTimeNanos);
> for (;;) {
> - long timeoutMillis = (selectDeadLineNanos - currentTimeNanos +
> 500000L) / 1000000L;
> + long timeoutNanos = selectDeadLineNanos - currentTimeNanos +
> 500000L;
> + long timeoutMillis = timeoutNanos / 1000000L;
> + System.out.printf("timeoutNanos: %d, timeoutMillis: %d |
> deadline: %d - now: %d | hastask: %d\n",
> + timeoutNanos, timeoutMillis,
> + selectDeadLineNanos, currentTimeNanos, hasTasks() ? 1 :
> 0);
> if (timeoutMillis <= 0) {
> if (selectCnt == 0) {
> int ready = Native.epollWait(epollFd.intValue(), events,
> 0);
> + System.out.printf("ready: %d\n", ready);
> if (ready > 0) {
> return ready;
> }
> {code}
> And this gives :
> {code}
> timeoutNanos: 1000500000, timeoutMillis: 1000 | deadline: 2001782341816510 -
> now: 2001781341816510 | hastask: 0
> timeoutNanos: 1000500000, timeoutMillis: 1000 | deadline: 2001782342087239 -
> now: 2001781342087239 | hastask: 0
> timeoutNanos: 1000500000, timeoutMillis: 1000 | deadline: 2001782342166947 -
> now: 2001781342166947 | hastask: 0
> timeoutNanos: 508459, timeoutMillis: 0 | deadline: 2001781342297987 - now:
> 2001781342289528 | hastask: 0
> ready: 0
> timeoutNanos: 508475, timeoutMillis: 0 | deadline: 2001781342357719 - now:
> 2001781342349244 | hastask: 0
> ready: 0
> timeoutNanos: 509327, timeoutMillis: 0 | deadline: 2001781342394822 - now:
> 2001781342385495 | hastask: 0
> ready: 0
> timeoutNanos: 509339, timeoutMillis: 0 | deadline: 2001781342430192 - now:
> 2001781342420853 | hastask: 0
> ready: 0
> timeoutNanos: 509510, timeoutMillis: 0 | deadline: 2001781342461588 - now:
> 2001781342452078 | hastask: 0
> ready: 0
> timeoutNanos: 509493, timeoutMillis: 0 | deadline: 2001781342495044 - now:
> 2001781342485551 | hastask: 0
> ready: 0
> {code}
> The nanosecond timeout all come from {{eventLoop.schedule(this, 10000,
> TimeUnit.NANOSECONDS);}} in {{Message::Flusher}}.
> Knowing that, I'm not sure what would be best to do, and I have a hard time
> understanding Message::Flusher, but to me it looks like trying to schedule
> less tasks would probably help and I didn't think anything obvious that could
> be done with netty.
> Changing {{if (++runsWithNoWork > 5)}} to 2 seems to help a little bit, but
> that isn't really significant.
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