> There were some complaints that the MIDAS sequencer was slow.  Specifically, the
> complaint was that even lines in the sequence that didn't do any (like COMMENT
> commands) tooks > 100ms to execute.  These slow sequencer steps could be a
> little annoying if a script had to change a large number of ODB variables before
> starting.
> 
> I tested this a little using a trivial sequence; note that I did all tests using
> mhttpd with mongoose enabled on a newer macbook pro.  I found that with the
> mongoose server each line in a sequencer script was taking ~100ms.  This is
> consistent with the loop in the main thread, which is only doing a cm_yield and
> a sleep:
> 
>    while (!_abort) {
>       status = ss_mutex_wait_for(request_mutex, 0);
>       status = cm_yield(0);
>       if (status == RPC_SHUTDOWN)
>          break;
>       sequencer();
>       status = ss_mutex_release(request_mutex);
>       ss_sleep(100);
>    }
> 
> I tested reducing the sleep to 20ms.  As expected, this made the sequencer more
> zippy, able to execute ~50 commands per second.
> 
> I tried to think what would be downsides to making this change.  I think that
> the main web communication should not be affected, because that communication is
> all handled by the separate mongoose thread.
> 
> I checked how much extra CPU was used if the sleep was reduced from 100ms to
> 20ms.  I found that when a sequence was not running the CPU increased from 0% to
> 0.2% with my change.  When a sequence was running the CPU increased from 0.8% to
> 4% with my change.  4% is a little high, though I'd say still reasonable.  I
> found that most of the CPU usage was occuring because every call to
> 'sequencer()' resulted in a call to db_set_record("/Sequencer/State"...).  I
> guess that making that call 50 times causes the somewhat heavy CPU usage.
> 
> I would argue that it would still be worth making that change, so that the
> sequencer can be more zippy.

The minimal time slice on most systems is 10 ms, and nothing prevents us from switching to
that. The original 100 ms was more for the fact that you can see the sequencer statements
executed one after the other (with the color bar). But this is more a "debugging" feature which 
we not really need. 

To do it "right" the sequencer would have to _return_ a sleep time. Like if it is in a wait loop (as
most of the time), the sleep time could be close to 1 second, to correctly update the wait
progress bar. If the sequencer executes ODB set statements, the wait time could be zero, so
thousands of statements can be executed in one second. The problem we will then have of course
that the sequencer will block the "request_mutex" almost always, which would prevent the
mongoose server from serving anything. So this should be carefully tested. It could be (on most OS)
that releasing the mutex by the main loop immediately switches to the mongoose thread, which would
make the web server still quite responsive, but I'm not sure about that. So as a first change making
the sleep time 10ms should be fine.

Stefan