On Thu, Oct 11, 2018 at 12:14 PM Nadav Har'El <n...@scylladb.com> wrote:
> On Thu, Oct 11, 2018 at 12:10 PM Pekka Enberg <penb...@scylladb.com> > wrote: > >> >> >> On Thu, Oct 11, 2018 at 11:50 AM Nadav Har'El <n...@scylladb.com> wrote: >> >>> >>> tests/misc-ctxsw.cc >>> >>> To run it in OSv you can do "scripts/build image=tests" and then >>> "scripts/run.py -e tests/misc-ctxsw.so" >>> >>> What this test does is to create two threads (on the same core, or on >>> two different cores) which alternate waking the other thread and then >>> waiting (via a condition variable). >>> Strangely I just tested this again on my laptop (which is newer than the >>> paper), and got (for the colocated case) in OSv is around 400ns, and in >>> Linux around 1800ns, so the difference is even more pronounced today. >>> >> >> Perhaps you are seeing the effects of Meltdown mitigation in Linux >> context switching paths? >> > > Perhaps, I didn't look into this carefully, I just ran it now to make sure > I was pointing the OP to the right test :-) > Does meltdown mitigation have a performance cost even when switching > between two threads belonging to the same process? > Yes, that's my understanding. Linux mitigates against Meltdown with kernel page table isolation. Userspace processes no longer have kernel memory mapped in the upper half of the address space (to avoid side-channel attacks from reading arbitrary mapped memory), which means that the kernel needs to switch page tables to access kernel memory. Search for SWITCH_TO_KERNEL_CR3 for system call entry in arch/x86/entry/entry_64.S, for example. - Pekka -- You received this message because you are subscribed to the Google Groups "OSv Development" group. To unsubscribe from this group and stop receiving emails from it, send an email to osv-dev+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
