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Groups > comp.os.linux.hardware > #2397
| From | Clemens Ladisch <clemens@ladisch.de> |
|---|---|
| Newsgroups | comp.os.linux.misc, comp.os.linux.hardware |
| Subject | Re: Realtime clock accessible without context-switch? |
| Date | 2014-05-22 20:36 +0200 |
| Message-ID | <bu6udeF1cbuU1@mid.individual.net> (permalink) |
| References | (1 earlier) <bu1qcoFt9amU1@mid.individual.net> <llhkkb$56l$1@dont-email.me> <bu46amFe2ebU1@mid.individual.net> <wwvegzm4v6d.fsf@l1AntVDjLrnP7Td3DQJ8ynzIq3lJMueXf87AxnpFoA.invalid> <llkadb$s96$1@dont-email.me> |
Cross-posted to 2 groups.
Charles T. Smith wrote: > On Wed, 21 May 2014 23:35:22 +0100, Richard Kettlewell wrote: >> Clemens Ladisch <clemens@ladisch.de> writes: >>> An I/O access (about one microsecond) is much slower than the SYSCALL >>> instruction; and with the RTC, you'd need to read several registers. >> >> As it happens, fetching the current time doesn’t involve a syscall >> anyway (on amd64). Yeah; the kernel reads the realtime clock once when booting, and then tries to avoid touching it ever again by using another clock to track the current time. > And I'm reasonably sure that doing a task switch on linux has much more > involved in it than just the trap instruction itself. On AMD64, a system call does not involve a trap; it's just a jump to a predefined address that also switches the privilege level so that the kernel code is allowed to access all memory pages. A task switch has to save and restore all registers, and to switch page tables if the new task is in another process. Regards, Clemens
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Realtime clock accessible without context-switch? "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2014-05-20 10:29 +0000
Re: Realtime clock accessible without context-switch? Clemens Ladisch <clemens@ladisch.de> - 2014-05-20 21:56 +0200
Re: Realtime clock accessible without context-switch? "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2014-05-21 07:29 +0000
Re: Realtime clock accessible without context-switch? Thomas Richter <thor@math.tu-berlin.de> - 2014-05-21 09:47 +0200
Re: Realtime clock accessible without context-switch? William Unruh <unruh@invalid.ca> - 2014-05-21 15:51 +0000
Re: Realtime clock accessible without context-switch? Clemens Ladisch <clemens@ladisch.de> - 2014-05-21 19:32 +0200
Re: Realtime clock accessible without context-switch? Richard Kettlewell <rjk@greenend.org.uk> - 2014-05-21 23:35 +0100
Re: Realtime clock accessible without context-switch? "Charles T. Smith" <cts.private.yahoo@gmail.com> - 2014-05-22 07:53 +0000
Re: Realtime clock accessible without context-switch? Clemens Ladisch <clemens@ladisch.de> - 2014-05-22 20:36 +0200
Re: Realtime clock accessible without context-switch? Richard Kettlewell <rjk@greenend.org.uk> - 2014-05-22 20:26 +0100
Re: Realtime clock accessible without context-switch? William Unruh <unruh@invalid.ca> - 2014-05-22 22:48 +0000
Re: Realtime clock accessible without context-switch? Jan Pieter <jp@jp.dhs.orig> - 2014-06-01 09:58 +0200
Re: Realtime clock accessible without context-switch? Todd <Todd@invalid.invalid> - 2014-05-23 14:20 -0700
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