Context switching problem

Right now my machine is essentially idle, and I get (first line deleted):

$ vmstat 5 procs memory swap io system cpu r b swpd free buff cache si so bi bo in cs us sy id wa 4 0 168 plus 120 45620 136772 3288768 0 0 0 30 114 243 100 0 0 0 5 0 168 plus 120 47516 136784 3288768 0 0 0 19 113 248 99 1 0 0 4 0 168 plus 120 47620 136796 3288768 0 0 0 44 185 524 99 1 0 0

Now if I start up a bit of serious processing,

procs memory swap io system cpu r b swpd free buff cache si so bi bo in cs us sy id wa 6 0 52180 25308 72116 3416636 377 6883 1630 68 plus 189 372 5647 84 16 0 0 6 0 52628 659984 72184 2784132 17 97 399 619 277 9592 89 11 0 0 7 0 52628 22212 72284 3415928 1 0 381 1268 plus 1 717 5605 82 18 0 0

Now on this machine, the cpu is busy essentially 100% of the time, but most of it is used up, at the moment, with BOINC projects that run just to consume otherwise unused CPU time; they run at nice level 19. They do extremely little io and just sit there computing, so they context switch at most when their time-slots are used up or when the kernel wants a processor for something else. They were running both in the first case, above, and the second. In the second, I started up program to populate a fairly small database. The dbms uses a large number (about two dozen) of processes, not all of which actually run at one time, but they run frequently, accounting for process switches. E.g., page fetchers (4), page cleaners (4), log reader, log writer, three "agents", a deadlock detector, ... . I also have two hyperthread Xeon processors, so it needs to context switch less as there there are more "compute servers."

It all depends on what you are doing. If you are just typing to a UseNet user agent program (such as Thunderbird, as I am doing), and no other busy processes are running, you would get very few context switches. But remember, lots of daemon processes are running in the background. I stopped that database stuff (about a dozen remain, but they are not doing anything), and on my machine there are about 136 processes in the process table but only about 5 are running. OTOH, if you were doing some complicated processing (or a lot of simple things at once), have only one single threaded processor, you can expect a lot of context switching. One nice thing about Linux is that it is actually quite good at process switching, so it is not the problem it once was.

In the earliest versions of the UNIX OS that I used (very early 1970s), they did context switches about every two seconds because it required the entire program and its data to be swapped out to disk and the next one to be read in. And with 2400rpm disk drives, and stepping motor head buttemblies with slow seek times, you could not afford to do that very often. Nowdays, we do not even swap anymore, but just page out those portions of RAM that are no longer in the working set. Also, to context switch, if you have enough memory, all you have to do is change some in-memory tables a bit, and some memory management registers in the hardware.

-- .~. Jean-David Beyer Registered Linux User 85642. V PGP-Key: 9A2FC99A Registered Machine 241939. ^^-^^ 07:50:00 up 8 days, 22:36, 5 users, load average: 4.37, 4.28, 4.17