On Thu, Aug 25, 2016 at 05:56:25PM +0000, Ben Maurer wrote:
rseq opens up a whole world of algorithms to userspace – algorithms
that are O(num CPUs) and where one can have an extremely fast fastpath
at the cost of a slower slow path. Many of these algorithms are in use
in the kernel today – per-cpu allocators, RCU, light-weight reader
writer locks, etc. Even in cases where these APIs can be implemented
today, a rseq implementation is often superior in terms of
predictability and usability (eg per-thread counters consume more
memory and are more expensive to read than per-cpu counters).
Isn’t the large number of uses of rseq-like algorithms in the kernel a
pretty substantial sign that there would be demand for similar
algorithms by user-space systems programmers?
Yes and no. It provides a substantial sign that such algorithms could
and should exist; however "someone should do this" doesn't demonstrate
that someone *will*. I do think we need a concrete example of a
userspace user with benchmark numbers that demonstrate the value of this
approach.
Mathieu, do you have a version of URCU that can use rseq to work per-CPU
rather than per-thread? URCU's data structures would work as a
benchmark.
Ben, Mathieu, Dave, do you have jemalloc benchmark numbers with and
without rseq? (As well as memory usage numbers for the reduced memory
usage of per-CPU pools rather than per-thread pools?)