On 4/19/24 02:45, Gernot Heiser via Devel wrote:
> On 19 Apr 2024, at 14:00, Andrew Warkentin <andreww...@gmail.com> wrote:
>>
>> Somehow I said Send()/Recv()/Reply() when I meant Call()/Recv()/Send()
>> (MCS of course doesn't even have a distinct Reply() function).
>
> the correct syscall to use is ReplyWait():
>
> Recv(…);
> while (1) {
> ReplyWait(…);
> }
>
> Es per my blog: Send() and Recv() should only ever be used in initialisation
> and exception handling.
>
>> The QNX/Linux equivalent is:
>>
>> int main()
>> {
>> char buf[100];
>> int f = open("/dev/null", O_WRONLY);
>> int i;
>> for (i = 0; i < 10; i++) {
>> uint64_t start = __rdtsc();
>> uint64_t end;
>> int j;
>> for (j = 0; j < 10000; j++){
>> if (write(f, buf, 100) != 100){
>> printf("cannot write\n");
>> exit(1);
>> }
>> }
>> end = __rdtsc();
>> printf("cycles: %u\n", end - start);
>> }
>> }
>
> so you’re using standard I/O to /dev/null
>
> My Posix is a bit rusty, but this should be buffered in the library (i.e.
> most calls will *not* result in a system call).
Not true on any *nix I am familiar with. The buffered version is fwrite().
write() always results in a real system call.
That said, I know nothing about QNX other than it being based on a microkernel.
It might be that QNX _does_ implement userspace buffering for write() and avoid
the system call overhead in this benchmark.
--
Sincerely,
Demi Marie Obenour (she/her/hers)
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