[self-interest] Self CPU usage and file handling

[Chris Double]

On Tue, Apr 22, 2014 at 9:07 PM, Russell Allen <mail at russell-allen.com> wrote:
>
> To reproduce: look at top(1) when ./vm/Self is running in another terminal.
> It always hovers at 5-10% CPU usage on my machine.

I did some investigation in a Linux Self system. If I run 'vm/Self'
with no snapshot so it's just in the interactive scanner I see it
floating between 0 and 1% CPU on Linux (3-5% running under strace). I
see the jumping in and out of the 'read' system call. The cpu usage
matches the following C program which was written to imitate what the
Self VM is doing at this point:

<http://pastebin.com/NGxgLZy1>

While low it would be good to remove this if possible. Can the
interactive scanner read loop be exited when it's no longer used? I
assume the SIGALRM usage is used for the Self scheduler. Perhaps that
could be optimized such that it's turned off if all processes are
blocked in i/o.

If I create a snapshot using 'worldBuilder.self' and run that without
entering the desktop (ie. staying in the REPL) I still see only 0-1%
CPU. Now it's entering/existing the 'select' system call though.

If I start the desktop (via 'desktop open), Self CPU usage jumps to
3-5%. The desktop overhead seems high. Here strace shows that
'gettimeofday' is called a lot, followed by 'select'. This is what I
get with about 60 seconds of idle in the desktop:

% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
 84.32    0.024001           7      3312      3263 select
  6.95    0.001979           0      6342           rt_sigprocmask
  5.46    0.001555           0      3590      3269 rt_sigreturn
  2.70    0.000768           0     15455           gettimeofday
  0.37    0.000104           0      3504      3469 recv
  0.13    0.000036           0       196           getrusage
  0.07    0.000020           0        64           mmap2
  0.00    0.000000           0       179         6 read
  0.00    0.000000           0        45         2 write
  0.00    0.000000           0        30         1 open
  0.00    0.000000           0        28           close

The 'gettimeofday' calls appear to come from real_time_prim in
miscPrims.cpp. This is the _TimeReal primitive and only used in
'currentTime' in 'traits time'. It's hard to test with that stubbed
out since it's used a lot all over the place (via 'current' in 'traits
time').

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