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Nicely written article with good detail about the problems in the older architectures. One thing that I'm still a little fuzzy about is on the Linux kernel side

Nicely written article with good detail about the problems in the older architectures. One thing that I'm still a little fuzzy about is on the Linux kernel side of things in the context of multi core multi socket. Which CPU does the kernel actually run on and how does it dispatch the process/threads on the different cores/sockets?

wila 10y ago
Yes, Frank has a very nice series going there. More parts are expected to follow. As to your question on what CPU the kernel normally runs on? AFAIK, it normally starts at core 0, processor 0. You can see for yourself where processes run. For example using the ps command. It will print kernel threads in square brackets and it can also list cpu and core number behind a /. Eg, example output of a linux VM with 2 processors. $ ps -ax PID TTY STAT TIME COMMAND 1 ? Ss 0:03 /sbin/init splash 2 ? S 0:00 [kthreadd] 3 ? S 0:00 [ksoftirqd/0] 5 ? S< 0:00 [kworker/0:0H] 7 ? S 0:15 [rcu_sched] 8 ? S 0:00 [rcu_bh] 9 ? S 0:00 [migration/0] 10 ? S 0:00 [watchdog/0] 11 ? S 0:00 [watchdog/1] etcetera.. removed the rest for readability The CPU scheduler then handles the rest to see on which thread/core to run a new process. This post might also help: http://superuser.com/questions/389161/what-do-the-mean-in-ps-and-how-do-you-create-a-process-like-it http://superuser.com/questions/389161/what-do-the-mean-in-ps...
angry_octet 10y ago
The simple answer is that the kernel runs on all cores. But the answer is really that it depends which part of the kernel you are talking about. Whenever user mode does a trap to supervisor mode it is running the top half of the kernel. An interrupt (device or timer) causes the core to run the bottom half of the kernel. Many systems used to only send these to cpu0, and it is typical that lots of network I/O would occur on cpu0. In the PIII era, with a wide bus shared by all cores, most kernel data structures were not multithreaded, i.e. single run queue, great big kernel locks. There are now many per-core data structures, which can be in local memory. See http://www.tldp.org/LDP/tlk/kernel/kernel.html http://www.tldp.org/LDP/tlk/kernel/kernel.html and for a recent discussion of scheduling: http://www.ece.ubc.ca/~sasha/papers/eurosys16-final29.pdf http://www.ece.ubc.ca/~sasha/papers/eurosys16-final29.pdf

wila 10y ago

Yes, Frank has a very nice series going there. More parts are expected to follow. As to your question on what CPU the kernel normally runs on? AFAIK, it normally starts at core 0, processor 0. You can see for yourself where processes run. For example using the ps command. It will print kernel threads in square brackets and it can also list cpu and core number behind a /. Eg, example output of a linux VM with 2 processors. $ ps -ax PID TTY STAT TIME COMMAND 1 ? Ss 0:03 /sbin/init splash 2 ? S 0:00 [kthreadd] 3 ? S 0:00 [ksoftirqd/0] 5 ? S< 0:00 [kworker/0:0H] 7 ? S 0:15 [rcu_sched] 8 ? S 0:00 [rcu_bh] 9 ? S 0:00 [migration/0] 10 ? S 0:00 [watchdog/0] 11 ? S 0:00 [watchdog/1] etcetera.. removed the rest for readability The CPU scheduler then handles the rest to see on which thread/core to run a new process. This post might also help: http://superuser.com/questions/389161/what-do-the-mean-in-ps-and-how-do-you-create-a-process-like-it http://superuser.com/questions/389161/what-do-the-mean-in-ps...

angry_octet 10y ago

The simple answer is that the kernel runs on all cores. But the answer is really that it depends which part of the kernel you are talking about. Whenever user mode does a trap to supervisor mode it is running the top half of the kernel. An interrupt (device or timer) causes the core to run the bottom half of the kernel. Many systems used to only send these to cpu0, and it is typical that lots of network I/O would occur on cpu0. In the PIII era, with a wide bus shared by all cores, most kernel data structures were not multithreaded, i.e. single run queue, great big kernel locks. There are now many per-core data structures, which can be in local memory. See http://www.tldp.org/LDP/tlk/kernel/kernel.html http://www.tldp.org/LDP/tlk/kernel/kernel.html and for a recent discussion of scheduling: http://www.ece.ubc.ca/~sasha/papers/eurosys16-final29.pdf http://www.ece.ubc.ca/~sasha/papers/eurosys16-final29.pdf