On Wed, Dec 09, 2009 at 09:07:33AM -0500, John Baldwin wrote: > On Thursday 26 November 2009 10:14:20 am Linda Messerschmidt wrote: > > It's not clear to me if this might be a problem with the superpages > > implementation, or if squid does something particularly horrible to > > its memory when it forks to cause this, but I wanted to ask about it > > on the list in case somebody who understands it better might know > > whats going on. :-) > > I talked with Alan Cox some about this off-list and there is a case that can > cause this behavior if the parent squid process takes write faults on a > superpage before the child process has called exec() then it can result in > superpages being fragmented and never reassembled. Using vfork() should > prevent this from happening. It is a known issue, but it will probably be > some time before it is addressed. There is lower hanging fruit in other > areas > in the VM that will probably be worked on first.
For me the whole threads puzzles me. Especially because vfork is often called a solution. Scenario A Parent with super page fork/exec This problem can happen because there is a race. The parent now has it's super pages fragmented permanently!? the child throws away his pages because of the exec!? Scenario B Parent with super page vfork/exec This problem won't happen because the child has no pseudo copy of the parents memory and then starts with a completely new map. Scenario C Parent with super page fork/ no exec The problem can happen because the child shares the same memory over it's complete lifetime. The parent can get it's super pages fragmented over time. I don't see a use case for scenario A, because vfork is there since over 16 years. I use fork myself, because it is easier sometimes, but people writing big programms such as squid should know better. If squid doesn't use vfork they likely have a reason. With scenario C I don't see how vfork can help, since this is not a legal case for vfork. I use quid myself, but don't know how it handles it's childs. But isn't the whole story about such slave childs that they share memory with the master? - How can vfork be solution for this case? How can fragmentation of super pages be avoided at all? I obviously don't have enough clue about this to understand those details. Hope that someone can enlighten me. -- B.Walter <be...@bwct.de> http://www.bwct.de Modbus/TCP Ethernet I/O Baugruppen, ARM basierte FreeBSD Rechner uvm. _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
