Messages in this thread | | | Date | Fri, 06 Jan 2012 12:31:38 +0000 | From | "Jan Beulich" <> | Subject | Re: [PATCH] x86-64: fix memset() to support sizes of 4Gb and above |
| |
>>> On 06.01.12 at 12:05, Ingo Molnar <mingo@elte.hu> wrote:
> * Jan Beulich <JBeulich@suse.com> wrote: > >> While currently there doesn't appear to be any reachable >> in-tree case where such large memory blocks may be passed to >> memset() (alloc_bootmem() being the primary non-reachable one, >> as it gets called with suitably large sizes in FLATMEM >> configurations), we have recently hit the problem a second >> time in our Xen kernels. Rather than working around it a >> second time, prevent others from falling into the same trap by >> fixing this long standing limitation. >> >> Signed-off-by: Jan Beulich <jbeulich@suse.com> > > Have you checked the before/after size of the hotpath? > > The patch suggests that it got shorter by 3 instructions: > >> - movl %edx,%r8d >> - andl $7,%r8d >> - movl %edx,%ecx >> - shrl $3,%ecx >> + movq %rdx,%rcx >> + andl $7,%edx >> + shrq $3,%rcx > > [...] > >> movq %rdi,%r10 >> - movq %rdx,%r11 >> > > [...] > >> - movl %r11d,%ecx >> - andl $7,%ecx >> + andl $7,%edx > > Is that quick impression correct? I have not tried building or > measuring it.
Yes, that's correct.
As the bodies of the individual flavors didn't change, I see no risk in this change causing any performance degradation.
> Would be nice to add support for arch/x86/lib/memset_64.S as > well, and look at the before/after performance of it. In > userspace we can do a lot more accurate measurements of this > kind:
I'll see whether I can get this done, but I admit that I'm entirely unfamiliar with this tool and its infrastructure. I hope doing this is not going to be a requirement for acceptance of the patch.
Jan
| |