Re: hammer: MAP_32BIT

[Timothy Miller <miller@techsource.com>]

H. Peter Anvin wrote:
> Timothy Miller wrote:
> 
>>>The purpose is that there is a slight task-switching speed advantage if
>>>the address is in the bottom 4 GB.  Since this affects every process,
>>>and most processes use very little TLS, this is worthwhile.
>>>
>>>This is fundamentally due to a K8 design flaw.
>>
>>Is there an explicit check somewhere for this?  Are the page tables laid
>>out differently?
>>
> 
> 
> No, there are two ways to load the FS base register: use a descriptor,
> which is limited to 4 GB but is faster, or WRMSR, which is slower, but
> unlimited.
> 


Ulrich Drepper wrote:
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> 
> Timothy Miller wrote:
> 
> 
>>Why does there ever need to be an explicit HINT that you would prefer a
>><32 bit address, when it's known a priori that <32 is better?  Why
>>doesn't the mapping code ALWAYS try to use 32-bit addresses before
>>resorting to 64-bit?
> 
> 
> Because not all memory is addressed via GDT entries.  In fact, almost
> none is, only thread stacks and similar gimicks.  If all mmap memory
> would by default be served from the low memory pool you soon run out of
> it and without any good reason.


All I have to say is... I appreciate your patience with my ignorant 
questions.  :)