serverid(s)_exist, critical path work.

[Volker Lendecke]

On Thu, Feb 07, 2013 at 10:40:48AM -0500, simo wrote:
> Well if you want to avoid the syscall you need to always consider that
> rogue writes may destroy memory locations. It's either/or, so I guess
> what you need to choose is whether you can accept the risk and do
> everything in user space using shared memory or similar techniques or
> whether the risk is too high and you need the kernel to act as arbiter.
> 
> I wonder if there is a way to set canaries from user space to prevent
> processes to accidentally clobber memory areas with large memcpy or
> similar ... maybe a clever setting of mmap regions ...
> 
> I've been using memory barrier and synchronization primitives to
> implement single writer multiple readers w/o locking, and robust mutexes
> basically do the same I think.
> 
> Maybe there is a way to combine robust mutexes for performance with a
> fallback to fcntl locks for recovering from very bad situations ?

Falling back to fcntl under high pressure in my tests made
the situation much worse due to the single system-wide
spinlock gating every single fcntl lock operation. I made
the reference to epoll because I could imagine this is a
mechanism not dying under 10.000 simultaneous events.

Do the mutexes, fall back to a locking daemon nicely
queueing stuff in user space, communicated to via epoll. We
just have to fill in the details ... :-)

We need to coordinate share modes and leases of 30.000
clients connected to a cluster all opening the share root
directory.

Volker

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