-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On Thursday 22 Nov 2007, Kazim Zaidi wrote: > On 22/11/2007, Raj Mathur <[EMAIL PROTECTED]> wrote: > [snip] > > > So to sum up: > > > > Times when the CPU is executing user code is counted in %user. > > Times when the CPU is executing a system call (or running any other > > kernel code) is counted in %system. > > Times when the CPU is waiting for some I/O to complete is counted > > in %iowait. > > I'm not sure I understand this. When a process requests I/O, it is > blocked (or removed from ready queue) and CPU moves on to execute > next process in the ready-to-run-queue. When the I/O completes, an > event wakes up the blocked process(es). > A process may be waiting for its I/O while CPU executing some user > code. Does this time counted in %user or %iowait?
When one process is waiting for I/O and another is running in (say) user state, the CPU is actually in user state. That time will be accounted for in %user, not in %iowait. > Times when the CPU is doing nothing is counted in %idle. > > > Since the CPU must be doing one of these things at any time, the > > sum of these percentages will always be 100. > > [snip] > > If %user and %iowait overlap, then why is the sum 100? > > Or, is %iowait the time when there are no processes in ready-queue > because all of them are waiting for I/O? I think that is idle time > and not iowait time. Nope, that is %iowait time. Idle is when no process is doing anything. If you want to see this in action, do the following on a relatively unloaded system: - - Open up one terminal and run: sar -u 1 10000 - - Open up another terminal and run: ls -lR / > /dev/null Watch the %iowait times in the sar output go up as ls keeps waiting to read the disk. Now start up some CPU-intensive process like glxgears (GNU Chess is even better) and watch the %iowait times go down. This is because the time ls is spending waiting for I/O to complete is being soaked up by glxgears/gnuchess using up the CPU. Regards, - -- Raju - -- Raj Mathur [EMAIL PROTECTED] http://kandalaya.org/ Freedom in Technology & Software || February 2008 || http://freed.in/ GPG: 78D4 FC67 367F 40E2 0DD5 0FEF C968 D0EF CC68 D17F PsyTrance & Chill: http://schizoid.in/ || It is the mind that moves -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.6 (GNU/Linux) iD8DBQFHRR4IyWjQ78xo0X8RAgiSAJ0T+DJdIV3jlUcFxgN0KI6idAsBEQCfbjdH zPzt1qH4Dwtc7+cgWeNYiKw= =zRlw -----END PGP SIGNATURE----- _______________________________________________ ilugd mailinglist -- ilugd@lists.linux-delhi.org http://frodo.hserus.net/mailman/listinfo/ilugd Next Event: http://freed.in - February 22/23, 2008 Archives at: http://news.gmane.org/gmane.user-groups.linux.delhi http://www.mail-archive.com/ilugd@lists.linux-delhi.org/
