serverid(s)_exist, critical path work.

[Volker Lendecke]

On Thu, Feb 07, 2013 at 09:16:56AM -0500, Ira Cooper wrote:
> epoll and friends are very scalable for what they are, but you are still
> taking a system call hit when you use them.  And at least on Solaris
> that'll kill you.

Ok, doing locking without syscalls, that's what futexes are
for under Linux at least I guess. But they are not 100%
robust, rogue memory writes could clobber the robust list.

At this moment I am more interested in graceful degradation.
Linux fcntl completely falls over when someone takes the
allrecord lock, thousands of waiters pile up an the
allrecord lock is dropped. This makes the machine completely
unusable for a while, sending the SMB clients into timeouts.
I had thought that I can get away without the allrecord
lock, but ctdb definitely depends on the ability to freeze
all access to the database. This together with the non 100%
robustness of robust mutexes makes them look less shiny than
they initially did.

> I'm not sure.  It needs measurement, I agree.  The merge to master is more
> so I can merge back to 3.6, and gather comments for that merge.  (I'd hope
> we can green light that once the review is done.)

You need that in an official 3.6? Can you make it optional?

Volker

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