[Gllug] performance of xen dom0 vs native linux

[Nix]

On 10 May 2009, James Courtier-Dutton verbalised:

> 2009/5/10 Richard Jones <rich at annexia.org>:
>>> On Sat, May 09, 2009 at 10:31:33PM +0100, James Courtier-Dutton wrote:
>> 2009/5/9 Richard Jones <rich at annexia.org>:
>>
>>> The memory management problems found in the x86 hardware can be
>>> mitigated by using software that does it better, e.g. The Java VM.
>>
>> This doesn't even make sense.  It has nothing to do with
>> virtualization.
>>
>> [Another random anecdote that has nothing to do with the subject snipped]
>
> Memory management/fragmentation is certainly a problem when one scales
> up the amount of virtual machines per physical x86 CPU. Your "It has
> nothing to do with virtualization" statement is just plain wrong.

Er, the virtual->physical mappings take every bit as long (and are every
bit as especially slow without EPT or something like it) whether the
physical mappings are fragmented or not. Userspace does not require
contiguous physical pages, and userspace pages can be moved about
freely, and are.

There is one possible source of slowdowns: glibc malloc makes use of
mmap() sometimes (for large allocations), which does trigger page table
adjustments. This *might* change speeds, but I wouldn't expect it would
be much: most allocations are small, and are suballocated by malloc()
from its own brk()-allocated arena, without needing to make any system
calls.
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