On Thu, Jul 31, 2008 at 01:33:50PM +0100, Jamie Lokier (jamie@xxxxxxxxxxxxx) wrote: > This is why marking the pages COW would be better. Automatic! > There's no need for a notification, merely letting go of the page > references - yes, the hardware / TCP acks already do that, no locking > or anything! :-) The last reference is nothing special, it just means > the next file write/truncate sees the count is 1 and doesn't need to > COW the page. It depends... COW can DoS the system: consider attacker who sends a page, writes there, sends again and so on in lots of threads. Depending on link capacity eventually COW will eat the whole RAM. > > There was a linux aio_sendfile() too. Google still knows about its > > numbers, graphs and so on... :) > > I vaguely remember it's performance didn't seem that good. <q> Benchmark of the 100 1MB files transfer (files are in VFS already) using sync sendfile() against aio_sendfile_path() shows about 10MB/sec performance win (78 MB/s vs 66-72 MB/s over 1 Gb network, sendfile sending server is one-way AMD Athlong 64 3500+) for aio_sendfile_path(). </q> So, it was really better that sync sendfile :) > One of the problems is you don't really want AIO all the time, just > when a process would block because the data isn't in cache. You > really don't want to be sending *all* ops to worker threads, even > kernel threads. And you preferably don't want the AIO interface > overhead for ops satisfied from cache. That's how all AIO should work of course. We are getting into a bit of offtopic, but aio_sendfile() worked that way as long as syslets, although the former did allocate some structures before trying to send the data. > Syslets got some of the way there, and maybe that's why they were > faster than AIO for some things. There are user-space hacks which are > a bit like syslets. (Bind two processes to the same CPU, process 1 > wakes process 2 just before 1 does a syscall, and puts 2 back to sleep > if 2 didn't wake and do an atomic op to prove it's awake). I haven't > tested their performance, it could suck. Looks scary :) Thread allocation in userspace is rather costly operations compared to syslet threads in kernelspace. But depending on IO pattern this may or may not be a noticeble factor... It requires testing and numbers. -- Evgeniy Polyakov -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
