On Sun, Jul 11, 2021, at 9:17 PM, vivek thakkar wrote:
> We ran a test to measure the syscall performance on two different
> kernels (v4.9.x and v4.0.9). The program is as simple as this:
>
> for (int i =0; i< 100'000'000; i++) {
> syscall(SYS_getpid);
> }
>
> The program was built for x86 and was using the "vdso" mechanism to
> generate the system call and we could confirm that it was
> transitioning into the kernel using sysenter.
>
> We find that the time taken by each system call takes 10ns more in
> 4.9.x as compared to 4.0.9. On deeper analysis, we found that there
> are 40 more instructions that get executed in the newer kernel version
> - the user space transitioning mechanism based off of vdso remains the
> same.
>
> commit 5f310f739b4cc343f3f087681e41bbc2f0ce902d
> Author: Andy Lutomirski <luto@xxxxxxxxxx>
> Date: Mon Oct 5 17:48:15 2015 -0700
>
> x86/entry/32: Re-implement SYSENTER using the new C path
>
> Is that something that is already known to the community?
>
Yes. We made a conscious decision to trade a bit of performance for a lot of maintainability, especially for a legacy-ish path. The old SYSENTER code was a mess.
> Regards,
> Vivek Thakkar
>
