Bananymous
9eb3834ae5
If the processor has invariant TSC it can be used to measure time. We keep track of the last nanosecond and TSC values and offset them based on the current TSC. This allows getting current time in userspace. The implementation maps a single RO page to every processes' address space. The page contains the TSC info which gets updated every 100 ms. If the processor does not have invariant TSC, this page will not indicate the capability for TSC based timing. There was the problem about how does a processor know which cpu it is running without doing syscall. TSC counters may or may not be synchronized between cores, so we need a separate TSC info for each processor. I ended up adding sequence of bytes 0..255 at the start of the shared page. When a scheduler gets a new thread, it updates the threads gs/fs segment to point to the byte corresponding to the current cpu. This TSC based timing is also used in kernel. With 64 bit HPET this probably does not bring much of a benefit, but on PIT or 32 bit HPET this removes the need to aquire a spinlock to get the current time. This change does force the userspace to not use gs/fs themselves and they are both now reserved. Other one is used for TLS (this can be technically used if user does not call libc code) and the other for the current processor index (cannot be used as kernel unconditionally resets it after each load balance). I was looking at how many times timer's current time was polled (userspace and kernel combined). When idling in window manager, it was around 8k times/s. When running doom it peaked at over 1 million times per second when loading and settled at ~30k times/s. |
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| include/LibELF | ||
| CMakeLists.txt | ||
| dummy.cpp | ||