Re: [PATCH 7/7] x86/intel_rdt: Add CAT documentation and usage guide

[Marcelo Tosatti]

On Tue, Mar 31, 2015 at 10:27:32AM -0700, Vikas Shivappa wrote:
> 
> 
> On Thu, 26 Mar 2015, Marcelo Tosatti wrote:
> 
> >
> >I can't find any discussion relating to exposing the CBM interface
> >directly to userspace in that thread ?
> >
> >Cpu.shares is written in ratio form, which is much more natural.
> >Do you see any advantage in maintaining the
> >
> >(ratio -> cbm bitmasks)
> >
> >translation in userspace rather than in the kernel ?
> >
> >What about something like:
> >
> >
> >		      root cgroup
> >		   /		  \
> >		  /		    \
> >		/		      \
> >	cgroupA-80			cgroupB-30
> >
> >
> >So that whatever exceeds 100% is the ratio of cache
> >shared at that level (cgroup A and B share 10% of cache
> >at that level).
> 
> But this also means the 2 groups share all of the cache ?
> 
> Specifying the amount of bits to be shared lets you specify the
> exact cache area where you want to share and also when your total
> occupancy does not cover all of the cache. For ex: it gets more
> complex when you want to share say only the left quarter of the
> cache. cgroupA gets left half and cgroup gets left quarter. The
> bitmask aligns with how the h/w is designed to share the cache which
> gives you flexibility to define any specific overlapping areas of
> the cache.

> >https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/6/html/Resource_Management_Guide/sec-cpu_and_memory-use_case.html
> >
> >cpu â the cpu.shares parameter determines the share of CPU resources
> >available to each process in all cgroups. Setting the parameter to 250,
> >250, and 500 in the finance, sales, and engineering cgroups respectively
> >means that processes started in these groups will split the resources
> >with a 1:1:2 ratio. Note that when a single process is running, it
> >consumes as much CPU as necessary no matter which cgroup it is placed
> >in. The CPU limitation only comes into effect when two or more processes
> >compete for CPU resources.
> >
> >
> 
> These are more defined in terms of how many cache lines (or how many
> cache ways) they can use and would be difficult to define them in
> terms of percentage. In contrast the cpu share is a time shared
> thing and is much more granular where as here its not , its
> occupancy in terms of cache lines/ways.. (however this is not really
> defined as a restriction but thats the way it is now).
> Also note that the granularity of the bitmasks define the
> granularity of the percentages and in some SKUs the granularity is
> 2b and not 1b.. So technically you wont be able to even allocate
> percentage of cache even in 10% granularity for most of the cases
> (if there are 30MB and 25 ways like in one of hsw SKU) and this will
> vary for different SKUs which makes it more complicated for users.
> However the user library is free to define own interface based on
> the underlying cgroup interface say for example you never care about
> the overlapping and using it for a specific SKU etc.. The underlying
> cgroup framework is meant to be  generic for all SKus and used for
> most of the use cases.
> 
> Also at this point I see a lot of enterprise and and other users
> already using the cgroup interface or shown interest in the same.
> However I see your point where you indicate the ease with which user
> can specify in size/percentage which he might be used to doing for
> other resources rather than bits where he needs to get an idea size
> by calculating it seperately - But again note that you may not be
> able to define percentages in many scenarios like the one above. And
> another question would be we would need to convince the users to
> adapt to the modified percentage user model (ex: like the one you
> say above where percentage - 100 is the one thats shared)
> I can review this requirements and others I have received and get
> back to see the closest that can be done if possible.
> 
> Thanks,
> Vikas

Vikas,

I see. Don't have anything against performing the translation in userspace
(i agree userspace should be able to allow ratios and specific
minimum/maximum counts). Can you please export the relevant information
in files in /sys or cgroups itself rather than requiring userspace to
parse CPUID etc? Including the EBX register from CPUID(EAX=10H, ECX=1),
which is necessary to implement "reserved LLC" properly.

The current interface is unable to handle the cross CPU case, though.
It would be necessary to expose per-socket masks. 

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