57 lines
2.6 KiB
Markdown
57 lines
2.6 KiB
Markdown
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# Environment variables
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Some environment variables can be used to adjust settings during the execution
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of libovni, they all begin with the `OVNI_` prefix. Be sure that all threads of
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the same node use the same environment variables.
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## OVNI_TMPDIR
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During the execution of your program, a per-thread buffer is kept where the new
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events are being recorded. When this buffer is full, it is written to disk and
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emptied, an operation known as flush. This may take a while depending on the
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underliying filesystem.
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Keep in mind that the thread will be blocked until the flush ends, so if your
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filesystem is slow it would interrupt the execution of your program for a long
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time. It is advisable to use the fastest filesystem available (see the tmpfs(5)
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and df(1) manual pages).
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You can select a temporary trace directory where the buffers will be flushed
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during the execution by setting the environment variable `OVNI_TMPDIR`. The last
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directory will be created if doesn't exist. In that case, as soon as a process
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calls `ovni_proc_fini()`, the traces of all its threads will be moved to the
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final directory at `$PWD/ovni`. Example:
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OVNI_TMPDIR=$(mktemp -u /dev/shm/ovni.XXXXXX) srun ./your-app
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To test the different filesystem speeds, you can use hyperfine and dd. Take a
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closer look at the max time:
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```
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$ hyperfine 'dd if=/dev/zero of=/gpfs/projects/bsc15/bsc15557/kk bs=2M count=10'
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Benchmark 1: dd if=/dev/zero of=/gpfs/projects/bsc15/bsc15557/kk bs=2M count=10
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Time (mean ± σ): 71.7 ms ± 130.4 ms [User: 0.8 ms, System: 10.2 ms]
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Range (min … max): 14.7 ms … 1113.2 ms 162 runs
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Warning: Statistical outliers were detected. Consider re-running this
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benchmark on a quiet PC without any interferences from other programs. It
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might help to use the '--warmup' or '--prepare' options.
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$ hyperfine 'dd if=/dev/zero of=/tmp/kk bs=2M count=10'
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Benchmark 1: dd if=/dev/zero of=/tmp/kk bs=2M count=10
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Time (mean ± σ): 56.2 ms ± 5.7 ms [User: 0.6 ms, System: 14.8 ms]
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Range (min … max): 45.8 ms … 77.8 ms 63 runs
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$ hyperfine 'dd if=/dev/zero of=/dev/shm/kk bs=2M count=10'
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Benchmark 1: dd if=/dev/zero of=/dev/shm/kk bs=2M count=10
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Time (mean ± σ): 11.4 ms ± 0.4 ms [User: 0.5 ms, System: 11.1 ms]
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Range (min … max): 9.7 ms … 12.5 ms 269 runs
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```
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## OVNI_TRACEDIR
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By default, the runtime trace will be placed in the `ovni` directory, inside the
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working directory. You can specify a different location to place the trace by
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setting the `OVNI_TRACEDIR` environment variable. It accepts a relative or
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absolute path, which will be created if it doesn't exist.
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