Re: [libc/string] State of PAGE_COPY_FWD / PAGE_COPY_THRESHOLD

[Maxim Kuvyrkov <maxim.kuvyrkov@linaro.org>]

> On Nov 10, 2016, at 11:48 AM, Andrew Pinski <pinskia@gmail.com> wrote:
> 
> On Wed, Nov 9, 2016 at 11:39 PM, Maxim Kuvyrkov
> <maxim.kuvyrkov@linaro.org> wrote:
>>> On Nov 1, 2016, at 5:59 PM, Adhemerval Zanella <adhemerval.zanella@linaro.org> wrote:
>>> 
>> ...
>>> $ cat sysdeps/x86/pagecopy.h
>>> 
>>> #define PAGE_SIZE           4096
>>> #define PAGE_COPY_THRESHOLD PAGE_SIZE
>>> 
>>> #define PAGE_COPY_FWD(dstp, srcp, nbytes_left, nbytes)  /* Implement it */
>>> 
>>> It should work on any other architecture as well.  Now the question
>>> is whether this actually does make sense for Linux.  Hurd/mach provided
>>> a syscall (?) to actually copy the pages (vm_copy) which seems to apply
>>> some tricks to avoid full copy pages. By 'linux zero page sharing' are
>>> you referring to KSM (Kernel Samepage Merging)?
>>> 
>>> If so, on a system without a provided kernel interface to work directed
>>> with underlying memory mapping (such as for mach), mem{cpy,set} will
>>> actually need to touch the pages and it will be up to kernel page fault
>>> mechanism to actually handle it (by identifying common pages and adjusting
>>> vma mapping accordingly). And AFAIK this are only enabled on KSM if you
>>> actually madavise the page explicit. So I am not grasping the need to
>>> actually implement page copying on Linux.
>> 
>> Linux kernel has a reserved page filled with zeroes, so it there /were/ a syscall to tell kernel to map N consecutive pages starting at address PTR to that zero page, we could use that in GLIBC for really big memset(0).
>> 
>> A quick investigation shows that there is no such syscall provided by the Linux kernel.  Doesn't mean we can't ask for / implement one.
> 
> And then there would be a COW interrupt on the first write.  Not a
> good idea.  Since most likely you are writing zeros to a big page for
> security reasons before filling it again with other data.

I'm looking at this as a possible performance optimization for a well-known benchmark.  

>   That mean
> each page would need to be copied which is normally slower than
> zeroing in the first place.

It may be like you say, or it may be a significant performance improvement.  I want to see numbers before deciding on how useful this may be.

> 
> COW is only useful when most of the pages will not be written to; it
> is not useful when doing memcpy or memset.  Mainly because you don't
> need to take the overhead of taking an interrupt twice (a system call
> is still an interrupt).


--
Maxim Kuvyrkov
www.linaro.org