GSoC: Avoiding syscall overhead

[Israel Jacques]

Hello,

I have sent a copy to one of the people listed as technical support (atillio@).

My name is Israel Jacques and I'm an undergrad at the University of
California, Berkeley. I am interested in working on removing overhead
from the 'SETPROCTITLE(3)' library routine. I see that in the ideas
page work has already begun to make this optimization. However,
searching through the mailing lists I have not found much discussion
on it.


After looking through the archives (see references), I have some
(crude) high level ideas as to how to approach the problem.

The idea, as suggested is to have a shared page between the kernel and
a userland process. Having such a shared page would avoid having to
invoke sysctl() which causes a the large overhead of a context switch.

The shared page
1. needs to be pinned as to avoid having it be paged out.
2. needs to stay alive for the entire duration of the process.
3. needs to be unique for every process.

Initially, I was thinking that a shared page should be created for ALL
processes, but that would expose vital information to all other
processes. Furthermore, any other process can alter the information of
another process.

As suggested in [9], allocating memory and reserving some region of
the process address space, pinning it, and setting the correct page
permissions (permissions in the PTE?) would be enough to create such a
shared page.

Though, letting the process know of this page would require a system
call. Again, suggested in reference [9] would add a section in the ELF
which would then have a global variable of (potentially) a pointer to
a struct with the process information.

Of course, a process could mistakenly use the pointer to access
outside the bounds of the page and cause trouble. Furthermore, checks
need to be made to insure that the process itself doesn't corrupt the
information in the struct.

Also, for every access a mutex would be needed as to avoid possible
race conditions between both the kernel and the process.

I'll be looking to go through:
src/sys/vm
src/lib/libc/gen/setproctitle.c
src/sys/kern/sysv_shm.c
...and the VM portion of the "4.4BSD" book.
(I just received my FreeBSD book today!)

References:
[1] http://lists.freebsd.org/pipermail/freebsd-hackers/2009-October/029750.html
[2] http://lists.freebsd.org/pipermail/freebsd-performance/2006-June/002038.html
[3] http://lists.freebsd.org/pipermail/freebsd-stable/2006-October/029983.html
[4] http://lists.freebsd.org/pipermail/freebsd-stable/2008-January/039392.html
[5] http://lists.freebsd.org/pipermail/freebsd-performance/2008-January/003107.html
[6] http://lists.freebsd.org/pipermail/freebsd-hackers/2008-March/023875.html
[7] http://lists.freebsd.org/pipermail/freebsd-hackers/2005-November/014452.html
[8] http://lists.freebsd.org/pipermail/freebsd-hackers/2011-February/034419.html
[9] http://lists.freebsd.org/pipermail/freebsd-hackers/2005-November/014453.html