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From "Paul J. Reder" <rede...@raleigh.ibm.com>
Subject [Fwd: [Patch]: Scoreboard as linked list.]
Date Fri, 03 Aug 2001 13:28:17 GMT
Unfortunately, the first version of the reply went directly to Ryan. Here is a copy...

Ryan Bloom wrote:
>
> -1.  As I have stated multiple times, if this uses a mutex to lock the list whenever
something
> walks the scoreboard, I can't accept it.  It will kill the performance for modules that
I have.

I'm very impressed that you were able to get the patch, apply it, build it and test it
in the 30 minutes between my post and yours :)

I do understand your concern. But I feel you are overreacting. When we had this discussion
you indicated this concern, but indicated a willingness to test the code if I wrote it.
It is now written, I would appreciate you actually taking a look at it before panning it
outright.

There are three times when locking can occur:
   1. When workers or processes come and go.
   2. When p_i_s_m checks for idleness.
   3. When the scoreboard is walked for status.

1 occurs rarely (even in my pathological test scenario of MRPC = 3000 under extreme abuse).
In fact, 1 can be eliminated almost entirely if you set MRPC to 0. The exception being the
replacement of segfaulted processes. Due to its infrequency and the short duration of the
locking time this should not cause any problem, and in my testing did not seem to since my
solution performed at least as well as yours.

2 occurs once per second. This is a quick walk to check the status flag and tally some
counts. This could be optimized in several ways *if* it proves to be a problem.
     A) It could avoid locking if MRPC = 0.
     B) It could use a read lock to permit simultaneous status walks.
     C) It could use finer granularity locking to allow multiple walkers withing the SB.
This code is already optimized, compared to the current code, since it only walks through
the workers in the active list. Workers on the free list aren't walked, therefore do not
incurr that overhead.

3 may occur frequently (in your case), or infrequently (in everyone elses ;) ). Locking
occurs in two parts. This can be optimized in a number of ways *if* it proves to be a
problem.
     A-C) As above.
     D) The walking code itself could be optimized to do less work during the locked phase.
        For example, the mod_status code currently does all of its network writes in the
        loop while holding a lock. The code could be rewritten to memcpy the parts into
        a local buffer, then write the buffer to the network outside the locking region.

Obviously we have a difference of opinion. The only way to resolve this is to test it.
I have done my part in the testing process. Vetoing it without testing it is bad form.

-- 
Paul J. Reder
-----------------------------------------------------------
"The strength of the Constitution lies entirely in the determination of each
citizen to defend it.  Only if every single citizen feels duty bound to do
his share in this defense are the constitutional rights secure."
-- Albert Einstein

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