Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

["Michael K. Edwards" <medwards.linux@gmail.com>]

Thanks for taking me at least minimally seriously, Alan.  Pretty
generous of you, all things considered.

On 2/23/07, Alan <alan@lxorguk.ukuu.org.uk> wrote:
> That example touches back into user space, but doesnt involve MMU changes
> or cache flushes, or tlb flushes, or floating point.

True -- on an architecture where a change of TLS does not
substantially affect the TLB and cache, which (AIUI) it does on most
or all ARMs.  (On a pre-EABI ARM, there is even a substantial
cache-related penalty for encoding the syscall number in the syscall
opcode, because you have to peek back at the text segment to see it,
which costs you a D-cache stall.)  Now put an sprintf with a %d in it
between a couple of the syscalls, and _your_ arch is hurting.  Deny
the userspace programmer the use of the FPU in threadlets, and they
become a lot less widely applicable -- and a lot flakier in a
non-wizard's hands, given that people often cheat around the small
number of x86 integer registers by using FP registers when copying
memory in bulk.

> errno is thread specific if you use it but errno is as I said before
> entirely a C library detail that you don't have to suffer if you don't
> want to. Avoiding that saves a segment register load - which isn't too
> costly but isn't free.

On your arch, it's a segment register -- and another
who-knows-how-many pages to migrate along with the stack and pt_regs.
On ARM, it's a coprocessor register that is incorrectly emulated by
most JTAG emulators (so bye-bye JTAG-assisted debugging and
profiling), or possibly a register stolen from the general purpose
register set.  On some MIPSes I have known you probably can't
implement TLS safely without a cache flush.

If you tell people up front not to touch TLS in threadlets -- which
means not to use routines from <stdlib.h> and <stdio.h> -- then
implementors may have enough flexibility to make them perform well on
a wide range of architectures.  Alternately, if there are some things
that threadlet users will genuinely need TLS for, you can tell them
that all of the threadlets belonging to process X on CPU Y share a TLS
context, and therefore things like errno can't be trusted across a
syscall -- but then you had better make fairly sure that threadlets
aren't preempted by other threadlets in between syscalls.  Similar
arguments apply to FPU state.

IEEE 754.  Harp, harp.  :-)

Cheers,
- Michael
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