1382844 - IonCompile has different behavior under rr

Status: RESOLVED INVALID

(Core :: JavaScript Engine: JIT, enhancement, P5)

Description

[Ted Campbell [:tcampbell]]

Currently rr emulates sysconf(_SC_NPROCESSORS_ONLN) as indicating 1 CPU. This causes Ion to use compile with different heuristics making to hard to capture traces of real-world behavior.

I'm running in to this because the bytecode limits for IonCompile and Ion inlining are very different between single-core and multi-core mode.

I'd like to add an ion option to override the js::GetCPUCount() values used to make heuristic decisions to allow captures on rr.

Comment 1

[Ted Campbell [:tcampbell]]

js::SetFakeCPUCount() already exists but it is only available under jsshell. Allowing an environment variable in full browser would probably be cleanest way to do this.

Comment 2

[Ted Campbell [:tcampbell]]

Attachment: 0001-Bug-1382844-Add-JS_FAKECPUCOUNT-environment-variable.patch

Add JS_FAKECPUCOUNT environment variable to initialize GlobalHelperThreadState().

Also fix the one case js where we use GetCPUCount() directly instead of HelperThreadState().

Comment 3

[Nicolas B. Pierron [:nbp]]

Comment on attachment 8889640 [details] [diff] [review]
0001-Bug-1382844-Add-JS_FAKECPUCOUNT-environment-variable.patch

Review of attachment 8889640 [details] [diff] [review]:
-----------------------------------------------------------------

Enabling chaos mode in rr (record -h) randomize the number of cores between 1 to 8.

Thus, if this patch is needed for cases where the chaos mode is not desirable, r=me with the following nit fixed and open an issue against rr to add a command line option for that.
Otherwise close this bug.

::: js/src/vm/HelperThreads.cpp
@@ +866,5 @@
> +    if (const char* env = getenv("JS_FAKECPUCOUNT")) {
> +        char * str_end;
> +        long fakeCpuCount = strtol(env, &str_end, 10);
> +        if (fakeCpuCount > 0 && fakeCpuCount < 10000 && (*str_end == '\0'))
> +            cpuCount = fakeCpuCount;

nit: 9999 cores, really?  I know some computers might have larger numbers of cores, but does it matter much yet?  What about:

  if (*env > '0' && *env <= '9')
      cpuCount = env[0] - '0';

Comment 4

[Ted Campbell [:tcampbell]]

I'm not sure if I still want to land this. Will have to revisit when I next need to deal with rr and jit behavior.