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Re: fork speed

Tim Newsham wrote:
> Hi,
>    I've been doing some profiling of the fork() call in cygwin.dll
> to find out why it is so slow and to see what could be done to speed
> it up.  I am using a pentium 133mhz with 32 megs of ram.  My
> observations
> are:
> >From fork() entry to parent exit averages about 50 msec.
> from fork entry to child exit averages about 75 msec.
> 75 msec is a lot of computer time.  I broke down the time a
> bit.  The biggest events are:
>      enter fork
>      CreateProcess  ----->
>      do some copying
>      resume child   ----->           child enters crt0
>      wait for child                  child longjmps to fork code
>                     <-----           signal parent to continue
>      more copying                    wait for parent
>      return                          init uinfo
>                                      init socket
>                                      return
> the parent takes about 10msec doing the initial steups until waiting
> for the child.  The child enters crt0 about 45msec after the
> fork started and quickly longjmps to the fork code and signals
> the parent to continue (at about 47msec the parent continues).
> The parent then finishes its work at about 50msec.  The child
> side gets signalled by the parent at around 50msec.  It then
> initializes uinfo until 65msec after fork, and initializes sockets
> until about 75msec after the fork.  Then it returns at about
> 75msec after the fork.
> I have yet to look into where the time goes when doing the
> CreateProcess.  This seems to be taking up about 35msec, roughly
> half the fork time, between when CreateProcess is called and
> the child actually starts running.
> I did look into the child initialization functions to init
> uinfo and init sockets a bit though.
> The init uinfo function reads in /etc/passwd and /etc/group and
> figures out what the user id is for the current process.  This
> seems a bit wasteful to me.  Is it ever the case that a fork()
> operation is going to change the uid and username of a user?
> I took out the call to the uinfo init function from the fork
> call and changed the passwd and group functions to do initialization
> (reading /etc/passwd and /etc/group) on first lookup (ie.
> getpwname()).  This reduced the fork time by about 15msec.
> The init socket function simply calls the winsock initialization
> function.  Again I opted for lazy evaluation and took the
> call out of the fork code and put it at the start of many
> of the socket functions.  This may make socket operations
> slightly slower but speeds up fork by about 10msec.  Since
> most programs dont use sockets this seems to be a big win to
> me.  As a nice side affect, this should get rid of problems people
> have been having with winsock dialing on demand each time
> an cygwin program is started up.  With my changes it should only
> do this when actually socket operations are used.
> So with these two pretty minor changes my fork() went from
> about 75msec to about 50msec, a 33% speed increase.  The difference
> is quite noticeable when running shell scripts and lots
> of shell commands (i.e. makefiles).
> I am still looking into other possible improvements so I'll defer
> posting patches until its all finished and I have done more complete
> testing.  If anyone has ideas on other possible improvements to
> fork please let me know.   Btw, is there a developers mailing
> list that this would be more appropriate for?  It seems that
> most people on the general cygwin list are probably not very
> interested in things like this.

Speaking for myself, I am very interested in things like this! Observing
the work of more expert persons has (I hope!) improved both my
understanding of Win32 programming issues and approaches for dealing
with them. So please, keep the list informed.

>                                       Tim N.
> -
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Don Sharp

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