Re: missing madvise functionality

[Eric Dumazet <dada1@cosmosbay.com>]

Ulrich Drepper a écrit :
> Eric Dumazet wrote:
>> Database workload, where the user multi threaded app is constantly
>> accessing GBytes of data, so L2 cache hit is very small. If you want to
>> oprofile it, with say a CPU_CLK_UNHALTED:5000 event, then find_vma() is
>> in the top 5.
> 
> We did have a workload with lots of Java and databases at some point
> when many VMAs were the issue.  I brought this up here one, maybe two
> years ago and I think Blaisorblade went on and looked into avoiding VMA
> splits by having mprotect() not split VMAs and instead store the flags
> in the page table somewhere.  I don't remember the details.
> 
> Nothing came out of this but if this is possible it would be yet another
> way to avoid mmap_sem locking, right?
> 

I was speaking about oprofile needs, that may interfere with target process 
needs, since oprofile calls find_vma() on the target process mm and thus zap 
its mmap_cache.

oprofile is yet another mmap_sem user, but also a mmap_cache destroyer.

We could at least have a separate cache, only for oprofile.

If done correctly we might avoid taking mmap_sem when the same vm_area_struct 
contains EIP/RIP snapshots.