Document environment variables

doc/user/runtime/env.md

@@ -0,0 +1,56 @@
+# Environment variables
+
+Some environment variables can be used to adjust settings during the execution
+of libovni, they all begin with the `OVNI_` prefix. Be sure that all threads of
+the same node use the same environment variables.
+
+## OVNI_TMPDIR
+
+During the execution of your program, a per-thread buffer is kept where the new
+events are being recorded. When this buffer is full, it is written to disk and
+emptied, an operation known as flush. This may take a while depending on the
+underliying filesystem.
+
+Keep in mind that the thread will be blocked until the flush ends, so if your
+filesystem is slow it would interrupt the execution of your program for a long
+time. It is advisable to use the fastest filesystem available (see the tmpfs(5)
+and df(1) manual pages).
+
+You can select a temporary trace directory where the buffers will be flushed
+during the execution by setting the environment variable `OVNI_TMPDIR`. The last
+directory will be created if doesn't exist. In that case, as soon as a process
+calls `ovni_proc_fini()`, the traces of all its threads will be moved to the
+final directory at `$PWD/ovni`. Example:
+
+	OVNI_TMPDIR=$(mktemp -u /dev/shm/ovni.XXXXXX) srun ./your-app
+
+To test the different filesystem speeds, you can use hyperfine and dd. Take a
+closer look at the max time:
+
+```
+$ hyperfine 'dd if=/dev/zero of=/gpfs/projects/bsc15/bsc15557/kk bs=2M count=10'
+Benchmark 1: dd if=/dev/zero of=/gpfs/projects/bsc15/bsc15557/kk bs=2M count=10
+  Time (mean ± σ):      71.7 ms ± 130.4 ms    [User: 0.8 ms, System: 10.2 ms]
+  Range (min … max):    14.7 ms … 1113.2 ms    162 runs
+ 
+  Warning: Statistical outliers were detected. Consider re-running this
+  benchmark on a quiet PC without any interferences from other programs. It
+  might help to use the '--warmup' or '--prepare' options.
+
+$ hyperfine 'dd if=/dev/zero of=/tmp/kk bs=2M count=10'
+Benchmark 1: dd if=/dev/zero of=/tmp/kk bs=2M count=10
+  Time (mean ± σ):      56.2 ms ±   5.7 ms    [User: 0.6 ms, System: 14.8 ms]
+  Range (min … max):    45.8 ms …  77.8 ms    63 runs
+ 
+$ hyperfine 'dd if=/dev/zero of=/dev/shm/kk bs=2M count=10'
+Benchmark 1: dd if=/dev/zero of=/dev/shm/kk bs=2M count=10
+  Time (mean ± σ):      11.4 ms ±   0.4 ms    [User: 0.5 ms, System: 11.1 ms]
+  Range (min … max):     9.7 ms …  12.5 ms    269 runs
+```
+
+## OVNI_TRACEDIR
+
+By default, the runtime trace will be placed in the `ovni` directory, inside the
+working directory. You can specify a different location to place the trace by
+setting the `OVNI_TRACEDIR` environment variable. It accepts a relative or
+absolute path, which will be created if it doesn't exist.

mkdocs.yml

@@ -29,6 +29,7 @@ nav:
       - user/concepts/trace-model.md
     - 'Runtime':
       - user/runtime/index.md
+      - user/runtime/env.md
       - user/runtime/mark.md
       - user/runtime/distributed.md
       - user/runtime/kernel.md