Date: Fri, 25 Jul 2008 11:46:18 -0400 From: John Baldwin <jhb@freebsd.org> To: Jeff Roberson <jroberson@jroberson.net> Cc: attilio@freebsd.org, arch@freebsd.org, ivmaykov@gmail.com Subject: Re: witness performance improvements Message-ID: <200807251146.19058.jhb@freebsd.org> In-Reply-To: <20080724162733.B954@desktop> References: <20080718163231.B954@desktop> <200807211141.09387.jhb@freebsd.org> <20080724162733.B954@desktop>
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On Thursday 24 July 2008 10:30:36 pm Jeff Roberson wrote: > On Mon, 21 Jul 2008, John Baldwin wrote: > > > On Friday 18 July 2008 10:41:58 pm Jeff Roberson wrote: > >> Hello, > >> > >> I have a patch that improves witness performance available at: > >> > >> http://people.freebsd.org/~jeff/witness.diff > >> > >> This improvement comes at the cost of some significant space overhead. It > >> changes the witness graph from a linked tree to a matrix based approach. > >> Relationships can be quickly resolved with a table lookup. The table size > >> is WITNESS_COUNT^2, or 1MB with the current count of 1024. > > > > Woo! Thanks for polishing this. > > > >> This patch also makes struct witness objects persistent even after the > >> last lock using this name has been removed. This is helpful for short > >> lived objects which may be created frequently. > > > > Originally, the idea was that if one had a LOR bug in a driver, one could > > kldunload the driver and have WITNESS forget about any orders for the > > driver's lock, fix the bug, and try again, but the short-lived names problem > > is much more common in practice, and trying to remove info about a specific > > lock class from the graph is a bit tenuous, so I think this is the better > > approach going forward. > > > >> To reduce lock contention on SMP witness_checkorder() now runs without the > >> w_mtx when there are no lock violations. I also cache a lock_list_entry > >> in each thread as allocating these requires the w_mtx. The entry is > >> disposed of at thread_exit(). > > > > Neat. > > > >> I'm mostly interested in hearing what people have to say about the space > >> bloat. I believe it is in a commit ready state. > > > > I think the space usage is perfectly fine. Also, now that you malloc the > > actual witness objects instead of putting them in the BSS (something that > > should have been done anyway I think), I would make the number of witness > > objects a loader tunable. > > Well, I'm glad there is a consensus that this is the right way forward. > The state of the code is that there may be a bug in the dot output but > I've not had any problems with the regular witness operation. > > There are still some style bugs in it. Attilio has expressed some > interest in a full review and style clean-up. I'd like to get what I have > now, minus the dot output, into svn. And then do a set of follow on > commits to add back dot or Attilio's comma separated graph output that can > be parsed to dot. > > Any objections to commiting this knowing it has some style bugs and a > little work left? I'd like to get people testing the core functionality > more. We've sat on this patch for a couple of years now as well. I think you can commit and work on the other stuff afterwards. I had noticed a few style things too but don't want those to hold this up. -- John Baldwin
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