Re: [RFC][PATCH 6/6] automatic tuning applied to some kernel components

[Nadia Derbey <Nadia.Derbey@bull.net>]

Eric W. Biederman wrote:
> Nadia Derbey <Nadia.Derbey@bull.net> writes:
> 
> 
>>2) why autotuning:
>>There are at least 3 cases where it can be useful
>>. for workloads that are known to need a big amount of a given resource type
>>(say shared memories), but we don't know what the maximum amount needed will be
>>. to solve the case of multiple applications running on a single system, and
>>that need the same tunable to be adjusted to feet their needs
>>. to make a system correctly react to eventual peak loads for a given resource
>>usage, i.e. make it tune up *and down* as needed.
> 
> 
>>In all these cases, the akt framework will enable the kernel to adapt to
>>increasing / decreasing resource consumption:
>>1) avoid allocating "a priori" a big amount of memory that will be used only in
>>extreme cases. This is the effect of doing an "echo <huge_value>
>>
>>>/proc/sys/kernel/shmmni"
>>
>>2) the system will come back to the default values as soon as the peak load is
>>over.
> 
> 
> At least the ipc ones are supposed to be DOS limits not behavior
> modifiers.  I do admit from looking at the code that there are some
> consequences of increasing things like shmmni.  However I think we
> would be better off with  better data structures and implementations
> that remove these consequences than this autotuning of
> denial-of-service limits.
> 

I do not fully agree with you:
It is true that some ipc tunables play the role of DoS limits.
But IMHO the *mni ones (semmni, msgmni, shmmni) are used by the ipc 
subsystem to adapt its data structures sizes to what is being asked for 
through the tunable value. I think this is how they manage to take into 
account a new tunable value without a need for rebooting the system: 
reallocate some more memory on demand.

Now, what the akt framework does, is that it takes advantage of this 
concept of "on demand memory allocation" to replace a user (or a daemon) 
that would periodically check its ipcs consumptions and manually adjust 
the ipcs tunables: Doing this from the user space would imply a latency 
that makes it difficult to react fast enough to resources running out.

Now, talking about DoS limits, akt implements them in a sense: each 
tunable managed by akt has 3 attributes exported to sysfs:
. autotune: enable / disable auto-tuning
. min: min value the tunable can ever reach
. max: max value the tunable can ever reach

Enabling a sysadmin to play with these min and max values makes it 
possible to refine the dynamic adjustment, and avoid that the tunable 
reaches really huge values.

Regards,
Nadia