On Mon, 2009-11-16 at 08:22 +1000, Shane wrote: > On Sun, 2009-11-15 at 21:25 +0000, Bishop, Peter wrote: > > > I'm still a bit leery of the extra I/O, as that equates to elapsed > > time which is our enemy in this scenario. I may try a named pipe test > > on a small case just to see what happens but Leslie's small test does > > not augur well. > > Look at Leslies follow up post - I suspect the initial results were > invalid. > I don't understand why you think there will be "extra" I/O. Any reads > will go through the page cache. The data are referenced there - by both > (any) programs. The named pipe will stall waiting for the reader > process. Unless the cache is flushed and the data subsequently re-read, > there will be no more than 1 I/O. > If the files are small and recently created, they may even still be *in* > the cache. Unlikely if you follow the (zVM) recommendation to screw the > guest memory allocation down as much as possible. But again, the guest > may see them as real physical I/O (FSVO "real" in this context), and zVM > may still have the pages in storage. So no (disk) I/O. > > If the application is doing (synchronous) raw/direct I/O, ignore all the > above. All bets are off in that case. > > Shane ...
I think by I/O, the OP is saying that reading the file directly is done via a single read() or fread() or ... . Using a named pipe, the "cat" does this, but then does a write() or fwrite() to the pipe. And then his program does another read() or fread() to read the pipe. So, even if there is no physical I/O, there __is__ increased processor overhead in writing to and reading from the pipe. The question is "how much" extra overhead? With the program being SAS (which I think is CPU heavy most of the time), the percentage is likely small compared to the total CPU usage. -- John McKown (from home) Maranatha! <>< ---------------------------------------------------------------------- For LINUX-390 subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO LINUX-390 or visit http://www.marist.edu/htbin/wlvindex?LINUX-390
