Messages in this thread | | | Date | Mon, 9 Nov 2009 14:28:30 +0100 | From | Andi Kleen <> | Subject | Re: [PATCH 22/23] sysctl arm: Remove binary sysctl support |
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On Mon, Nov 09, 2009 at 04:41:44AM -0800, Eric W. Biederman wrote: > >> >> The glibc pthread code that uses sysctl has no problems if sys_sysctl > >> >> is gone. It both falls back to reading /proc/sys and it just controls > >> >> an optimization and the code works with either result. Been there, > >> >> done that. > >> > > >> > /proc/sys is much slower than sysctl though. So you made program startup > >> > slower. > >> > >> Not much slower, but slower. I just measured it in a case that favors > >> sysctl and the ration is about 5:2. Or sysctl is about 2.5x faster. > >> About 49usec for open/read/close on proc and 19usec for sysctl. > >> In my emulation it is a bit slower than that. > > > > That's not good. > > Why?
Because it slows down a very common path.
> > > > a) all > > b) common ones used by glibc and perhaps a few others only > > c) none > > > > I suspect most users would use (b), in fact (c) might be redundant > > if (b) is cheap enough (which it should be) > > Next year I expect b == c.
Only if you ignore ABI compatibility.
> > If you think we need something better additional patches to sysctl_binary.c > are welcome. Just add your custom fast path before the through the > VFS.
I think sysctl_binary should be just stripped down for common sysctls (at least with a CONFIG)
> > Hmm? There's plenty of glibc code that uses the binary sysctl. > > Where? I grepped glibc today. The only use in a recent glibc is > in glibc-ports for the support of arm inb/outb. The only other > use in older glibc was checking to see if we ran on an SMP kernel.
That older glibc is widely deployed. And it won't go away next year.
> >> Ultimately what drives me most is that people are still accidentally > >> adding binary sysctls, which no one uses or tests. For a recent > >> example see: > > > > Yes I agree new binary sysctls should just be deprecated right now. > > Good. The only way to keep new ones from showing up is to that I know > of is to move everything into an emulation layer. I have done the heavy > lifting there and everything is emulated using the exact same code.
I agree with that.
I just think you should have two flavours of emulation layer: full and "common sysctls". This can be probably done with the same code and some strategic ifdefs.
-Andi -- ak@linux.intel.com -- Speaking for myself only.
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