On giovedì 29 marzo 2007, Blaisorblade wrote: > On mercoledì 28 marzo 2007, Jeff Dike wrote: > > [ This patch needs to get into 2.6.21, as it fixes a serious bug > > introduced soon after 2.6.20 ] > > > > Commit 62f96cb01e8de7a5daee472e540f726db2801499 introduced per-devices > > queues and locks, which was fine as far as it went, but left in place > > a global which controlled access to submitting requests to the host. > > This should have been made per-device as well, since it causes I/O > > hangs when multiple block devices are in use. > > > > This patch fixes that by replacing the global with an activity flag in > > the device structure in order to tell whether the queue is currently > > being run. > > Finally that variable has a understandable name. However in a mail from > Jens Axboe, titled: > "Re: [uml-devel] [PATCH 06/11] uml ubd driver: ubd_io_lock usage fixup" , > with Date: Mon, 30 Oct 2006 09:26:48 +0100, he suggested removing this flag > > altogether, so we may explore this for the future: > > > Add some comments about requirements for ubd_io_lock and expand its > > > use. > > > > > > When an irq signals that the "controller" (i.e. another thread on the > > > host, which does the actual requests and is the only one blocked on I/O > > > on the host) has done some work, we call again the request function > > > ourselves (do_ubd_request).
> > > We now do that with ubd_io_lock held - that's useful to protect against > > > concurrent calls to elv_next_request and so on. > > Not only useful, required, as I think I complained about a year or more > > ago :-) > > > XXX: Maybe we shouldn't call at all the request function. Input needed > > > on this. Are we supposed to plug and unplug the queue? That code > > > "indirectly" does that by setting a flag, called do_ubd, which makes > > > the request function return (it's a residual of 2.4 block layer > > > interface). > > Sometimes you need to. I'd probably just remove the do_ubd check and > > always recall the request function when handling completions, it's > > easier and safe. > Anyway, the main speedups to do on the UBD driver are: > * implement write barriers (so much less fsync) - this is performance > killer n.1 > * possibly to use the new 2.6 request layout with scatter/gather I/O, and > vectorized I/O on the host > * while at vectorizing I/O using async I/O > * to avoid passing requests on pipes (n.2) - on fast disk I/O becomes > cpu-bound. > To make a different but related example, with a SpeedScale laptop, it's > interesting to double CPU frequency and observe tuntap speed double too. > (with 1GHz I get on TCP numbers like 150 Mbit/s - 100 Mbit/s, depending > whether UML trasmits or receives data; with 2GHz double rates). > Update: I now get 150Mbit / 200Mbit (Uml receives/Uml sends) at 1GHz, and > still the double at 2Ghz. > This is a different UML though. > * using futexes instead of pipes for synchronization (required for previous > one). I forgot one thing: remember ubd=mmap? Something like that could have been done using MAP_PRIVATE, so that write had still to be called explicitly but unchanged data was shared with the host. Once a page gets dirty but is then cleaned, sharing it back is difficult - but even without that good savings could be achievable. That's to explore for the very future though. -- Inform me of my mistakes, so I can add them to my list! Paolo Giarrusso, aka Blaisorblade http://www.user-mode-linux.org/~blaisorblade - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
