Jan Kiszka wrote:
[Daniel, I put you in the CC as you showed some interest in this topic.]
as I indicated a some weeks ago, I had a closer look at the code the
user space libs currently produce (on x86). The following considerations
are certainly not worth noticeable microseconds on GHz boxes, but they
may buy us (yet another) few micros on low-end.
First of all, there is some redundant code in the syscall path of each
skin service. This is due to the fact that the function code is
calculated based on the the skin mux id each time a service is invoked.
The mux id has to be shifted and masked in order to combine it with the
constant function code part - this could also easily happen
ahead-of-time, saving code and cycles for each service entry point.
Here is a commented disassembly of some simple native skin service which
only takes one argument.
460: 55 push %ebp
461: 89 e5 mov %esp,%ebp
463: 57 push %edi
464: 83 ec 10 sub $0x10,%esp
Loading the skin mux-id:
467: a1 00 00 00 00 mov 0x0,%eax
Loading the argument (here: some pointer)
46c: 8b 7d 08 mov 0x8(%ebp),%edi
Calculating the function code:
46f: c1 e0 10 shl $0x10,%eax
472: 25 00 00 ff 00 and $0xff0000,%eax
477: 0d 2b 02 00 08 or $0x800022b,%eax
Saving the code:
47c: 89 45 f8 mov %eax,0xfffffff8(%ebp)
47f: 53 push %ebx
Loading the arguments (here only one):
480: 89 fb mov %edi,%ebx
Restoring the code again, issuing the syscall:
482: 8b 45 f8 mov 0xfffffff8(%ebp),%eax
485: cd 80 int $0x80
487: 5b pop %ebx
488: 83 c4 10 add $0x10,%esp
48b: 5f pop %edi
48c: 5d pop %ebp
48d: c3 ret
Looking at this code, I also started thinking about inlining short and
probably heavily-used functions into the user code. This would save the
function prologue/epilogue both in the lib and the user code itself. For
sure, it only makes sense for time-critical functions (think of
mutex_lock/unlock or rt_timer_read). But inlining could be made optional
The best optimization for rt_timer_read() would be to do the
cycles-to-ns conversion in user-space from a direct TSC reading if the
arch supports it (most do). Of course, this would only be possible for
strictly aperiodic timing setups (i.e. CONFIG_XENO_OPT_PERIODIC_TIMING off).
For the rt_mutex_lock()/unlock(), we still need to refrain from calling
the kernel for uncontended access by using some Xeno equivalent of the
futex approach, which would suppress most of the incentive to
micro-optimize the call itself.
for the user by providing both the library variant and the inlined
version. The users could then select the preferred one by #defining some
control switch before including the skin headers.
Any thoughts on this? And, almost more important, anyone around willing
to work on these optimisations and evaluate the results? I can't ATM.
Quite frankly, I remember that I once had to clean up the LXRT inlining
support in RTAI 3.0/3.1, and this was far from being fun stuff to do.
Basically, AFAICT, having both inline and out-of-line support for
library calls almost invariably ends up to a maintenance nightmare of
some sort, e.g. depending whether to compile with gcc's optimization on
or not, which might be dictated by the fact that one also wants
(exploitable) debug information or not, and so on. Not to speak of the
fact that you end up having two implementations to maintain separately.
This said, only the figures would tell us if such inlining brings
something significant or not to the picture performance-wise on low-end
hw, so I'd be interested to see those first.
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