Re: [RFC patch 4/7] futex: Add support for attached futexes

["Carlos O'Donell" <carlos@redhat.com>]

On 04/03/2016 07:30 AM, Linus Torvalds wrote:
> On Sun, Apr 3, 2016 at 6:16 AM, Ingo Molnar <mingo@kernel.org> wrote:
>>
>> So an ABI distinction and offloading the decision to every single application that
>> wants to use it and hardcode it into actual application source code via an ABI is
>> pretty much the _WORST_ way to go about it IMHO...
>>
>> So how about this: don't add any ABI details, but make futexes auto-attached on
>> NUMA systems (and obviously PREEMPT_RT systems)?
> 
> I agree.
> 
> Do *not* make this a visible new ABI.

Agreed.

We had similar requests in glibc to add APIs to tweak the parameters of
the elision for locks backed by hardware transactional memory.

The person submitting the patches always thinks this is a great API
because it allows them to write tests to verify their own work (which
means it still might be useful for internal testing or developing
auto-tuning).

Users have no clue what to do with the API and worse the state space of
the parameters is immense. You can't possibly do any kind of sensible
optimization without knowing a lot about the hardware.

So no public API was ever added in glibc for pthread_mutex_lock elision
parameters. Either the parameters work by default or you have to post
patches to change the auto-tuning used internally in glibc.

> So automatically using a local hashtable (for private mutexes - I
> think people need to just accept that a shared mutex is more costly)
> according to some heuristic is definitely the way to go. And yes, the
> heuristic may be well be - at least to start - "this is a preempt-RT
> system" (for people who clearly care about having predictable
> latencies) or "this is actually a multi-node NUMA system, and I have
> heaps of memory".

Agreed.
 
> Then, add a tunable (for root, not per-futex) to allow people to tweak it.

Agreed.

-- 
Cheers,
Carlos.