Re: [PATCH v2] mm: Optimized hugepage zeroing & copying from user

[Prathu Baronia <prathu.baronia@oneplus.com>]

The 04/22/2020 12:19, Will Deacon wrote:
> 
> I wrote the silly harness below for the snippets given in [1] but I can't
> see any difference between the forwards and backwards versions on any arm64
> systems I have access to.
> 
> Will
> 
> --->8
> 
> #include <errno.h>
> #include <stdio.h>
> #include <stdlib.h>
> #include <string.h>
> #include <time.h>
> #include <unistd.h>
> 
> [...]
> int main(void)
> {
> 	void *buf;
> 	unsigned long long delta;
> 	struct timespec ts_start, ts_end;
> 
> 	if (posix_memalign(&buf, PAGE_SIZE, BUF_SZ)) {
> 		perror("posix_memalign()");
> 		return -1;
> 	}
> 
> 	memset(buf, 0xd, BUF_SZ);
> 
> [...]

With this exact test code I also didn't observe any significant difference between
forward and backwards versions on SM8150.
------------------------------------------------------------------------------
Output on 8150 under controlled conditions(CPU0 & 6 turned on, CPUs set to max
frequency and DDR set to performance governor):
------------------------------------------------------------------------------
Forwards: took 0.319658 seconds
Backwards: took 0.320983 seconds
------------------------------------------------------------------------------

But when I used malloc instead of posix_memalign because that was the big difference
between this and our test code, I observed significant difference between
forward and backwards version on SM8150.
------------------------------------------------------------------------------
Output on 8150 under controlled conditions(CPU0 & 6 turned on, CPUs set to max
frequency and DDR set to performance governor):
------------------------------------------------------------------------------
Forwards: took 0.323157 seconds
Backwards: took 0.581638 seconds
------------------------------------------------------------------------------

I don't know the implementation differences between posix_memalign and
malloc which might lead to these results.

-- 
Prathu Baronia
OnePlus RnD