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rwtime-nd_example.txt
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Examples of rwtime-nd.stp, the Linux SystemTap version. rwtime-nd.stp measures syscall read() and write() times, summarizing them as histograms. A process name can be specified. For example, to measure read() and write() latency from processes named "node": # ./rwtime-nd.stp node Tracing read/write syscalls for processes named "node"... Hit Ctrl-C to end. ^C syscall read latency (ns): value |-------------------------------------------------- count 512 | 0 1024 | 0 2048 | 2 4096 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 278 8192 | 1 16384 | 5 32768 | 0 65536 | 0 syscall write latency (ns): value |-------------------------------------------------- count 4096 | 0 8192 | 0 16384 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 135 32768 |@@ 8 65536 | 0 131072 | 0 Those reads and writes were very fast, between 4 and 32 microseconds. Here's an example with much higher latencies: processes named "bash": # ./rwtime-nd.stp bash Tracing read/write syscalls for processes named "bash"... Hit Ctrl-C to end. ^C syscall read latency (ns): value |-------------------------------------------------- count 256 | 0 512 | 0 1024 |@ 1 2048 |@ 1 4096 | 0 8192 | 0 16384 | 0 32768 |@ 1 65536 |@@@@@@ 6 131072 |@@@@@@@@@@@ 11 262144 | 0 524288 | 0 ~ 2097152 | 0 4194304 | 0 8388608 |@ 1 16777216 | 0 33554432 |@ 1 67108864 |@@@ 3 134217728 |@@@@@@@@@ 9 268435456 |@@@ 3 536870912 |@@ 2 1073741824 |@@ 2 2147483648 | 0 4294967296 |@ 1 8589934592 | 0 17179869184 | 0 syscall write latency (ns): value |-------------------------------------------------- count 512 | 0 1024 | 0 2048 |@@@@@@@@@@@@@@@@@@@@@ 21 4096 |@@@@@@@@@@@@@@@@@@@@@@ 22 8192 |@@@@@@@@@@@@@@@@@@ 18 16384 | 0 32768 | 0 The high read time, with a mode between 134 and 268 milliseconds, corresponds to inter-keystroke latency as the bash shell reads keystrokes.