Re: hammer: MAP_32BIT

[Ulrich Drepper <drepper@redhat.com>]

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Andi Kleen wrote:

> That's just an inadequate data structure. It does an linear search of the
> VMAs and you probably have a lot of them. Before you add kludges like this 
> better fix the data structure for fast free space lookup.

If you mean the code in arch_get_unmapped_area(), yes, this needs
fixing.  In fact, Ingo has already a patch which brings back the
performance of thread creation to what we had back in September/October.


> In some vendor kernels it's already in /proc/pid/mapped_base, but that is 
> quite costly to change. That would probably give you the best of both, Just 
> set it to a low value for the thread stacks and then reset it to the default.
> 
> I guess that would be the better solution for your stacks. 

Are you sure this is the best solution?  It means the mmap regions for
restricted 31/32 bit addresses and that for the normal, unrestricted
mapping is continuous.  This removes a lot of freedom in deciding where
the unrestricted mappings are best located and it would make programs
using threads have a very different memory layout.  Not that it should
make any difference; but I can here /them/ already scream that this
breaks applications.

My kernel-uninformed opinion would be to keep the settings separate.

Oh, and please rename MAP_32BIT to MAP_31BIT.  This will save nerves on
all sides.

- -- 
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Ulrich Drepper \    ,-----------------'   \ Mountain View, CA 94041 USA
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