Date: Sat, 26 Jul 2008 00:17:39 +0200 From: "Attilio Rao" <attilio@freebsd.org> To: "Jeff Roberson" <jroberson@jroberson.net> Cc: arch@freebsd.org, ivmaykov@gmail.com Subject: Re: witness performance improvements Message-ID: <3bbf2fe10807251517v73447626j90458ebcd5345eaf@mail.gmail.com> In-Reply-To: <20080724162733.B954@desktop> References: <20080718163231.B954@desktop> <200807211141.09387.jhb@freebsd.org> <20080724162733.B954@desktop>
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2008/7/25, Jeff Roberson <jroberson@jroberson.net>: > 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. Please go on and commit the code, delaying any further improvement. Thanks, Attilio -- Peace can only be achieved by understanding - A. Einstein
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