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 _______________________________________________ Interest mailing list Interest@qt-project.org http://lists.qt-project.org/mailman/listinfo/interest
