[Interest] Qt5, XCB and X11

[Thiago Macieira]

On Tuesday 15 July 2014 15:40:52 Rutledge Shawn wrote:
> Sounds like a good start, but with some limitations: it's done by periodic
> scanning; and Linux Documentation/vm/ksm.txt says
> 
>  KSM only operates on those areas of address space which an application
>  has advised to be likely candidates for merging, by using the madvise(2)
>  system call: int madvise(addr, length, MADV_MERGEABLE).
> …
>  Applications should be considerate in their use of MADV_MERGEABLE,
>  restricting its use to areas likely to benefit.  KSM's scans may use a lot
>  of processing power: some installations will disable KSM for that reason.
> 
> cat /sys/kernel/mm/ksm/run => 0: so it's disabled on my system by default
> 
> So we need to add madvise() calls to Qt to enable this, or is it done
> already on another layer?  The only place I found any is in
> src/3rdparty/pcre/sljit/sljitUtils.c.  But it sounds like the cost
> outweighs the benefit often enough that it has remained a cautious opt-in
> feature.

We should *not* add that to Qt.

The question is whether the user wants to do it. If they want to do it for the 
application they're shipping because they believe there's a good chance that 
the libraries they ship match the one in the operating system, they'll scan 
the process map (/proc/self/maps) and advise which read-only sections of the 
libraries they want to share.

I repeat: this is an application action. Qt shall not do it.

> Then I wonder if we could test, after getting it working, how different Qt
> has to be before it stops working. I don't suppose it's easy to track which
> de-duplicated pages came from which (ranges inside) files.

It's possible any small change will be enough to cause problems. You want to 
enable this only when there's a good chance that the library is identical, 
from the very same build.

> http://jak-linux.org/projects/hardlink/ claims to replace complete file
> duplicates with hard links; I will have to try it.  But that is apparently
> also a periodic-scanning solution.

That is a much better solution. If you replace the library with hardlinks or 
symlinks, then the kernel knows it's the same file and then the pages 
automatically get shared.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center