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copied!<p>Switching between processes (given you actually switch, not run them in parallel) by an order of oh-my-god.</p> <p>Trapping from userspace to kernelspace used to be done with a processor interrupt earlier. Around 2005 (don't remember the kernel version), and after a discussion on the mailing list where someone found that trapping was <strong>slower</strong> (in absolute measures!) on a high-end xeon processor than on an earlier Pentium II or III (again, my memory), they implemented it with a new cpu instruction <code>sysenter</code> (which had actually existed since Pentium Pro I think). This is done in the Virtual Dynamic Shared Object (vdso) page in each process (cat /proc/pid/maps to find it) IIRC.</p> <p>So, nowadays, a kernel trap is basically just a couple of cpu instructions, hence rather few cycles, compared to tenths or hundreds of thousands when using an interrupt (which is really slow on modern CPU's).</p> <p>A context switch between processes is heavy. It means storing all processor state (registers, etc) to RAM (at a magic memory location in the user process space actually, guess where!), in practice dirtying all cached memory in the cpu, and reading back the process state for the new process. It will (likely) have nothing still in the cpu cache from last time it ran, so each memory read will be a cache miss, and needed to be read from RAM. This is rather slow. When I was at the university, I "invented" (well, I did come up with the idea, knowing that there is plenty of dye in a CPU, but not enough cool if it's constantly powered) a cache that was infinite size although unpowered when unused (only used on context switches i.e.) in the CPU, and implemented this in Simics. Implemented support for this magic cache I called CARD (Context-switch Active, Run-time Drowsy) in Linux, and benchmarked rather heavily. I found that it could speed-up a Linux machine with lots of heavy processes sharing the same core with about 5%. This was at relatively short (low-latency) process time slices, though.</p> <p>Anyway. A context switch is still pretty heavy, while a kernel trap is basically free.</p> <p>Answer to at which memory location in user-space, for each process:</p> <blockquote class="spoiler"> <p> At address zero. Yep, the null pointer! You can't read from this entire page from user-space anyway :) This was back in 2005, but it's probably the same now unless the CPU state information has grown larger than a page size, in which case they might have changed the implementation.</p> </blockquote>

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