Re: KSM For All Via LD_PRELOAD?

Gordan Bobic
2010-06-09T10:31:42Z

On 06/09/2010 09:56 AM, Paolo Bonzini wrote:
>>> Or is this too crazy an idea?
>>
>> It should work. Note that the the malloced memory should be aligned in
>> order to get better sharing.
>
> Within glibc malloc large blocks are mmaped, so they are automatically
> aligned. Effective sharing of small blocks would take too much luck or
> too much wasted memory, so probably madvising brk memory is not too useful.
>
> Of course there are exceptions. Bitmaps are very much sharable, but not
> big. And some programs have their own allocator, using mmap in all
> likelihood and slicing the resulting block. Typically these will be
> virtual machines for garbage collected languages (but also GCC for
> example does this). They will store a lot of pointers in there too, so
> in this case KSM would likely work a lot for little benefit.
>
> So if you really want to apply it to _all_ processes, it comes to mind
> to wrap both mmap and malloc so that you can set a flag only for
> mmap-within-malloc... It will take some experimentation and heuristics
> to actually not degrade performance (and of course it will depend on the
> workload), but it should work.

Arguably, the way QEMU KVM does it for the VM's entire memory block
doesn't seem to be distinguishing the types of memory allocation inside
the VM, so simply covering all mmap()/brk() calls would probably do no
worse in terms of performance. Or am I missing something?

Gordan