[PATCH 4 of 6] plan9: prevent potential wait()

[Jeff Sickel]

A small number, less than 130 forked children, may be
beneficial on Plan 9 (see following test script).  But
under certain cases like cloning a large repository or
updating something that produces a significant number of
children, we end up with an OSError exception that will
cause hg to rollback the transaction and delete any
changes written to the work area.  So until we have
a solution that lets us fork() thousands of children
like a clone of cpython would do in the worker calculation,
we just punt and don't fork any.

acme# cat children.py
#!/bin/python
import sys, os
import errno

def usage():
    help = "usage: children.py N\n"
    help += "   N is the number of forked childred you want to wait for in the run.\n"
    print help
    sys.exit(1)

def run(n):
    pids = []
    rfd, wfd = os.pipe()
    
    for i in range(n):
        pid = os.fork()
        if pid == 0:
            # print 'child %d' % (os.getpid())
            os._exit(0)
        pids.append(pid)
    
    for i in pids:
        try:
            w = os.wait()
        except OSError, e:
            print e

if __name__ == '__main__':
    args = len(sys.argv)
    if args == 1:
        print "starting 10 children"
        run(10)
    elif args == 2:
        n = sys.argv[1]
        if n.isdigit():
            print "starting %s children" % (n)
            run(int(n))
            sys.exit(0)
        usage()
    else:
        usage()






acme# for(i in `{seq 128 132}) python children.py $i
starting 128 children
starting 129 children
starting 130 children
[Errno 6] No children
starting 131 children
[Errno 6] No children
[Errno 6] No children
starting 132 children
[Errno 6] No children
[Errno 6] No children
[Errno 6] No children



On Aug 13, 2013, at 3:58 PM, Augie Fackler <raf at durin42.com> wrote:

> On Mon, Aug 12, 2013 at 06:01:10PM -0500, Jeff Sickel wrote:
>> # HG changeset patch
>> # User Jeff Sickel <jas at corpus-callosum.com>
>> # Date 1376347561 18000
>> #      Mon Aug 12 17:46:01 2013 -0500
>> # Branch stable
>> # Node ID b55c76e1f4c1726e2a537efa1b99c360626ac23d
>> # Parent  c92381647a5b63b85355bf62a60df8d13fc5f858
>> plan9: prevent potential wait()
> 
> Not sure I understand what the value of this is - how does this wait()
> for you, rather than being a (trivial, stupidly cheap) arithmetic
> operation?
> 
>> 
>> diff -r c92381647a5b -r b55c76e1f4c1 mercurial/worker.py
>> --- a/mercurial/worker.py	Mon Aug 12 17:44:31 2013 -0500
>> +++ b/mercurial/worker.py	Mon Aug 12 17:46:01 2013 -0500
>> @@ -48,6 +48,9 @@
>> def worthwhile(ui, costperop, nops):
>>     '''try to determine whether the benefit of multiple processes can
>>     outweigh the cost of starting them'''
>> +    # this trivial calculation does not benefit plan 9
>> +    if sys.platform == 'plan9':
>> +        return 0
>>     linear = costperop * nops
>>     workers = _numworkers(ui)
>>     benefit = linear - (_startupcost * workers + linear / workers)
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