Demonstrations of runqlat, the Linux eBPF/bcc version.
This program summarizes scheduler run queue latency as a histogram, showing
how long tasks spent waiting their turn to run on-CPU.
Here is a heavily loaded system:
# ./runqlat
Tracing run queue latency... Hit Ctrl-C to end.
^C
usecs : count distribution
0 -> 1 : 233 |*********** |
2 -> 3 : 742 |************************************ |
4 -> 7 : 203 |********** |
8 -> 15 : 173 |******** |
16 -> 31 : 24 |* |
32 -> 63 : 0 | |
64 -> 127 : 30 |* |
128 -> 255 : 6 | |
256 -> 511 : 3 | |
512 -> 1023 : 5 | |
1024 -> 2047 : 27 |* |
2048 -> 4095 : 30 |* |
4096 -> 8191 : 20 | |
8192 -> 16383 : 29 |* |
16384 -> 32767 : 809 |****************************************|
32768 -> 65535 : 64 |*** |
The distribution is bimodal, with one mode between 0 and 15 microseconds,
and another between 16 and 65 milliseconds. These modes are visible as the
spikes in the ASCII distribution (which is merely a visual representation
of the "count" column). As an example of reading one line: 809 events fell
into the 16384 to 32767 microsecond range (16 to 32 ms) while tracing.
I would expect the two modes to be due the workload: 16 hot CPU-bound threads,
and many other mostly idle threads doing occasional work. I suspect the mostly
idle threads will run with a higher priority when they wake up, and are
the reason for the low latency mode. The high latency mode will be the
CPU-bound threads. More analysis with this and other tools can confirm.
A -m option can be used to show milliseconds instead, as well as an interval
and a count. For example, showing three x five second summary in milliseconds:
# ./runqlat -m 5 3
Tracing run queue latency... Hit Ctrl-C to end.
msecs : count distribution
0 -> 1 : 3818 |****************************************|
2 -> 3 : 39 | |
4 -> 7 : 39 | |
8 -> 15 : 62 | |
16 -> 31 : 2214 |*********************** |
32 -> 63 : 226 |** |
msecs : count distribution
0 -> 1 : 3775 |****************************************|
2 -> 3 : 52 | |
4 -> 7 : 37 | |
8 -> 15 : 65 | |
16 -> 31 : 2230 |*********************** |
32 -> 63 : 212 |** |
msecs : count distribution
0 -> 1 : 3816 |****************************************|
2 -> 3 : 49 | |
4 -> 7 : 40 | |
8 -> 15 : 53 | |
16 -> 31 : 2228 |*********************** |
32 -> 63 : 221 |** |
This shows a similar distribution across the three summaries.
A -p option can be used to show one PID only, which is filtered in kernel for
efficiency. For example, PID 4505, and one second summaries:
# ./runqlat -mp 4505 1
Tracing run queue latency... Hit Ctrl-C to end.
msecs : count distribution
0 -> 1 : 1 |* |
2 -> 3 : 2 |*** |
4 -> 7 : 1 |* |
8 -> 15 : 0 | |
16 -> 31 : 25 |****************************************|
32 -> 63 : 3 |**** |
msecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 2 |** |
4 -> 7 : 0 | |
8 -> 15 : 1 |* |
16 -> 31 : 30 |****************************************|
32 -> 63 : 1 |* |
msecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 28 |****************************************|
32 -> 63 : 2 |** |
msecs : count distribution
0 -> 1 : 1 |* |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 27 |****************************************|
32 -> 63 : 4 |***** |
[...]
For comparison, here is pidstat(1) for that process:
# pidstat -p 4505 1
Linux 4.4.0-virtual (bgregg-xxxxxxxx) 02/08/2016 _x86_64_ (8 CPU)
08:56:11 AM UID PID %usr %system %guest %CPU CPU Command
08:56:12 AM 0 4505 9.00 3.00 0.00 12.00 0 bash
08:56:13 AM 0 4505 7.00 5.00 0.00 12.00 0 bash
08:56:14 AM 0 4505 10.00 2.00 0.00 12.00 0 bash
08:56:15 AM 0 4505 11.00 2.00 0.00 13.00 0 bash
08:56:16 AM 0 4505 9.00 3.00 0.00 12.00 0 bash
[...]
This is a synthetic workload that is CPU bound. It's only spending 12% on-CPU
each second because of high CPU demand on this server: the remaining time
is spent waiting on a run queue, as visualized by runqlat.
Here is the same system, but when it is CPU idle:
# ./runqlat 5 1
Tracing run queue latency... Hit Ctrl-C to end.
usecs : count distribution
0 -> 1 : 2250 |******************************** |
2 -> 3 : 2340 |********************************** |
4 -> 7 : 2746 |****************************************|
8 -> 15 : 418 |****** |
16 -> 31 : 93 |* |
32 -> 63 : 28 | |
64 -> 127 : 119 |* |
128 -> 255 : 9 | |
256 -> 511 : 4 | |
512 -> 1023 : 20 | |
1024 -> 2047 : 22 | |
2048 -> 4095 : 5 | |
4096 -> 8191 : 2 | |
Back to a microsecond scale, this time there is little run queue latency past 1
millisecond, as would be expected.
Now 16 threads are performing heavy disk I/O:
# ./runqlat 5 1
Tracing run queue latency... Hit Ctrl-C to end.
usecs : count distribution
0 -> 1 : 204 | |
2 -> 3 : 944 |* |
4 -> 7 : 16315 |********************* |
8 -> 15 : 29897 |****************************************|
16 -> 31 : 1044 |* |
32 -> 63 : 23 | |
64 -> 127 : 128 | |
128 -> 255 : 24 | |
256 -> 511 : 5 | |
512 -> 1023 : 13 | |
1024 -> 2047 : 15 | |
2048 -> 4095 : 13 | |
4096 -> 8191 : 10 | |
The distribution hasn't changed too much. While the disks are 100% busy, there
is still plenty of CPU headroom, and threads still don't spend much time
waiting their turn.
A -P option will print a distribution for each PID:
# ./runqlat -P
Tracing run queue latency... Hit Ctrl-C to end.
^C
pid = 0
usecs : count distribution
0 -> 1 : 351 |******************************** |
2 -> 3 : 96 |******** |
4 -> 7 : 437 |****************************************|
8 -> 15 : 12 |* |
16 -> 31 : 10 | |
32 -> 63 : 0 | |
64 -> 127 : 16 |* |
128 -> 255 : 0 | |
256 -> 511 : 0 | |
512 -> 1023 : 0 | |
1024 -> 2047 : 0 | |
2048 -> 4095 : 0 | |
4096 -> 8191 : 0 | |
8192 -> 16383 : 1 | |
pid = 12929
usecs : count distribution
0 -> 1 : 1 |****************************************|
2 -> 3 : 0 | |
4 -> 7 : 1 |****************************************|
pid = 12930
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 1 |****************************************|
32 -> 63 : 0 | |
64 -> 127 : 1 |****************************************|
pid = 12931
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 1 |******************** |
8 -> 15 : 0 | |
16 -> 31 : 0 | |
32 -> 63 : 0 | |
64 -> 127 : 0 | |
128 -> 255 : 0 | |
256 -> 511 : 0 | |
512 -> 1023 : 2 |****************************************|
pid = 12932
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 0 | |
32 -> 63 : 0 | |
64 -> 127 : 0 | |
128 -> 255 : 1 |****************************************|
256 -> 511 : 0 | |
512 -> 1023 : 1 |****************************************|
pid = 7
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 426 |************************************* |
4 -> 7 : 457 |****************************************|
8 -> 15 : 16 |* |
pid = 9
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 425 |****************************************|
8 -> 15 : 16 |* |
pid = 11
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 10 |****************************************|
pid = 14
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 8 |****************************************|
4 -> 7 : 2 |********** |
pid = 18
usecs : count distribution
0 -> 1 : 414 |****************************************|
2 -> 3 : 0 | |
4 -> 7 : 20 |* |
8 -> 15 : 8 | |
pid = 12928
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 1 |****************************************|
8 -> 15 : 0 | |
16 -> 31 : 0 | |
32 -> 63 : 0 | |
64 -> 127 : 1 |****************************************|
pid = 1867
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 15 |****************************************|
16 -> 31 : 1 |** |
32 -> 63 : 0 | |
64 -> 127 : 0 | |
128 -> 255 : 4 |********** |
pid = 1871
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 2 |****************************************|
16 -> 31 : 0 | |
32 -> 63 : 0 | |
64 -> 127 : 0 | |
128 -> 255 : 0 | |
256 -> 511 : 0 | |
512 -> 1023 : 1 |******************** |
pid = 1876
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 1 |****************************************|
16 -> 31 : 0 | |
32 -> 63 : 0 | |
64 -> 127 : 0 | |
128 -> 255 : 0 | |
256 -> 511 : 1 |****************************************|
pid = 1878
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 3 |****************************************|
pid = 1880
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 3 |****************************************|
pid = 9307
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 1 |****************************************|
pid = 1886
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 1 |******************** |
8 -> 15 : 2 |****************************************|
pid = 1888
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 3 |****************************************|
pid = 3297
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 1 |****************************************|
pid = 1892
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 1 |******************** |
32 -> 63 : 0 | |
64 -> 127 : 0 | |
128 -> 255 : 0 | |
256 -> 511 : 0 | |
512 -> 1023 : 2 |****************************************|
pid = 7024
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 4 |****************************************|
pid = 16468
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 3 |****************************************|
pid = 12922
usecs : count distribution
0 -> 1 : 1 |****************************************|
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 1 |****************************************|
16 -> 31 : 1 |****************************************|
32 -> 63 : 0 | |
64 -> 127 : 1 |****************************************|
pid = 12923
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 1 |******************** |
8 -> 15 : 0 | |
16 -> 31 : 0 | |
32 -> 63 : 0 | |
64 -> 127 : 2 |****************************************|
128 -> 255 : 0 | |
256 -> 511 : 0 | |
512 -> 1023 : 1 |******************** |
1024 -> 2047 : 1 |******************** |
pid = 12924
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 2 |******************** |
8 -> 15 : 4 |****************************************|
16 -> 31 : 1 |********** |
32 -> 63 : 0 | |
64 -> 127 : 0 | |
128 -> 255 : 0 | |
256 -> 511 : 0 | |
512 -> 1023 : 0 | |
1024 -> 2047 : 1 |********** |
pid = 12925
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 0 | |
32 -> 63 : 0 | |
64 -> 127 : 1 |****************************************|
pid = 12926
usecs : count distribution
0 -> 1 : 0 | |
2 -> 3 : 1 |****************************************|
4 -> 7 : 0 | |
8 -> 15 : 1 |****************************************|
16 -> 31 : 0 | |
32 -> 63 : 0 | |
64 -> 127 : 0 | |
128 -> 255 : 0 | |
256 -> 511 : 0 | |
512 -> 1023 : 1 |****************************************|
pid = 12927
usecs : count distribution
0 -> 1 : 1 |****************************************|
2 -> 3 : 0 | |
4 -> 7 : 1 |****************************************|
A -L option will print a distribution for each TID:
# ./runqlat -L
Tracing run queue latency... Hit Ctrl-C to end.
^C
tid = 0
usecs : count distribution
0 -> 1 : 593 |**************************** |
2 -> 3 : 829 |****************************************|
4 -> 7 : 300 |************** |
8 -> 15 : 321 |*************** |
16 -> 31 : 132 |****** |
32 -> 63 : 58 |** |
64 -> 127 : 0 | |
128 -> 255 : 0 | |
256 -> 511 : 13 | |
tid = 7
usecs : count distribution
0 -> 1 : 8 |******** |
2 -> 3 : 19 |******************** |
4 -> 7 : 37 |****************************************|
[...]
And a --pidnss option (short for PID namespaces) will print for each PID
namespace, for analyzing container performance:
# ./runqlat --pidnss -m
Tracing run queue latency... Hit Ctrl-C to end.
^C
pidns = 4026532870
msecs : count distribution
0 -> 1 : 40 |****************************************|
2 -> 3 : 1 |* |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 0 | |
32 -> 63 : 2 |** |
64 -> 127 : 5 |***** |
pidns = 4026532809
msecs : count distribution
0 -> 1 : 67 |****************************************|
pidns = 4026532748
msecs : count distribution
0 -> 1 : 63 |****************************************|
pidns = 4026532687
msecs : count distribution
0 -> 1 : 7 |****************************************|
pidns = 4026532626
msecs : count distribution
0 -> 1 : 45 |****************************************|
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 0 | |
32 -> 63 : 0 | |
64 -> 127 : 3 |** |
pidns = 4026531836
msecs : count distribution
0 -> 1 : 314 |****************************************|
2 -> 3 : 1 | |
4 -> 7 : 11 |* |
8 -> 15 : 28 |*** |
16 -> 31 : 137 |***************** |
32 -> 63 : 86 |********** |
64 -> 127 : 1 | |
pidns = 4026532382
msecs : count distribution
0 -> 1 : 285 |****************************************|
2 -> 3 : 5 | |
4 -> 7 : 16 |** |
8 -> 15 : 9 |* |
16 -> 31 : 69 |********* |
32 -> 63 : 25 |*** |
Many of these distributions have two modes: the second, in this case, is
caused by capping CPU usage via CPU shares.
USAGE message:
# ./runqlat -h
usage: runqlat.py [-h] [-T] [-m] [-P] [--pidnss] [-L] [-p PID]
[interval] [count]
Summarize run queue (scheduler) latency as a histogram
positional arguments:
interval output interval, in seconds
count number of outputs
optional arguments:
-h, --help show this help message and exit
-T, --timestamp include timestamp on output
-m, --milliseconds millisecond histogram
-P, --pids print a histogram per process ID
--pidnss print a histogram per PID namespace
-L, --tids print a histogram per thread ID
-p PID, --pid PID trace this PID only
examples:
./runqlat # summarize run queue latency as a histogram
./runqlat 1 10 # print 1 second summaries, 10 times
./runqlat -mT 1 # 1s summaries, milliseconds, and timestamps
./runqlat -P # show each PID separately
./runqlat -p 185 # trace PID 185 only