On Wed, Apr 07, 2004 at 12:34:55PM -0700, Andrew Morton wrote: > Andy Isaacson <[EMAIL PROTECTED]> wrote: > > Would there be any reason to allow O_DIRECT on the read side? > > Sure. It saves CPU,
OK, I can see that one. But it seems like a pretty small benefit to me -- CPU utilization is already really low. > avoids blowing pagecache, Um, that sounds like a bad idea to me. It seems to me it's the kernel's responsibility to figure out "hey, looks like a streaming read - let's not blow out the buffer cache trying to hold 20GB on a 512M system." If you're saying that the kernel guys have given up and the established wisdom is now "you gotta use O_DIRECT if you don't want to throw everything else out due to streaming data", well... I'm disappointed. > just as with O_DIRECT writes. Wouldn't opening both if= and of= with O_DIRECT turn dd into a synchronous copier? That would really suck in the "dd if=/dev/hda1 of=/dev/hdc1" case. With buffer cache doing readahead, that command can get, say, 40MB/s read and 40MB/s write; with synch read and synch write, it would drop to 40MB/s read+write, assuming that block sizes are big enough to amortize seek overhead. Having O_DIRECT on just of=, I think you can get back to 40MB/s+40MB/s. I claim that O_DIRECT on of= is important because you just plain *can not* do the minimal-sized IDE block scrub without it. I don't yet see a similar benefit to O_DIRECT on if= side. -andy _______________________________________________ Bug-coreutils mailing list [EMAIL PROTECTED] http://mail.gnu.org/mailman/listinfo/bug-coreutils
