Multiple page size support on FreeBSD?

[Andrew Duane]

Like all "performance" items (especially VM), it depends on the hardware and the load. On systems with small TLBs it helps more than with large TLBs. With software that needs access to lots of different areas the TLB gets more traffic so large ones help more. The answer for your firefox browser box with i386 is probably different from my compilation engine running MIPS, or his web server running AMD.

Back at Digital, we spent a lot of time trying to find the "one true answer" to superpages, only to discover there wasn't one. We ended up with a combination of automatic use from big allocations, a rarely used API to advise for big TLBs, and some background work that coalesced when possible.

 ....................................
Andrew L. Duane
Resident Architect - AT&T Technical Lead
m   +1 603.770.7088
o   +1 408.933.6944 (2-6944)
skype: andrewlduane
aduane at juniper.net



-----Original Message-----
From: owner-freebsd-hackers at freebsd.org [mailto:owner-freebsd-hackers at freebsd.org] On Behalf Of Alfred Perlstein
Sent: Wednesday, April 10, 2013 4:00 PM
To: Benjamin Kaduk
Cc: Wojciech Puchar; Sebastian Feld; freebsd-hackers at freebsd.org
Subject: Re: Multiple page size support on FreeBSD?

On 4/10/13 11:42 AM, Benjamin Kaduk wrote:
> On Wed, 10 Apr 2013, Wojciech Puchar wrote:
>
>>> How do your tests work?  Do you examine PTEs directly to check for 
>>> superpages or are you relying on the vm.pmap.pde sysctls?
>>
>> the later.
>>
>> anyway - algorithm described on list - that heuristics detects 
>> consecutive page access doesn't really help the urgent case - RANDOM 
>> access to large amount of memory.
>
> The algorithm is not a heuristic based on consecutive accesses, 
> promotion occurs when the entire superpage's worth of memory has 
> actually been accessed.  If I remember correctly, the performance gain 
> from superpages was only a few percent, so spending more time trying 
> to decide when to use them would make the algorithm a net wash.
>
> You should really watch the talk I linked to if you're interested, it 
> was quite interesting.
>
>> sequential access will get minimal improvement.
>>
>> IMHO the only way that really make sens is to add options to madvise 
>> to give kernel information about usage.
>
> Maybe.

It is cool that FreeBSD got this work via Alan Cox and the others that contributed.

I am wondering if it makes sense to have an explicit model.

At one place, for a platform with high performance but a very small TLB, we made it possible to explicitly request a large TLB for our process and it made a BIG difference for performance.

Sometimes being "general purpose" means that you can expose such low level things for the user to tune instead of requiring them to fit the app to a heuristic that may change.

-Alfred


>
> -Ben Kaduk
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