Re: [PATCH 1/3] ib core: Make device counter infrastructure dynamic

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Tue, May 17, 2016 at 01:34:34PM -0400, Doug Ledford wrote:
> > I noticed this too, but for sysfs reading I just felt it doesn't
> > matter.
> 
> I prefer not to have O(n^2), even for sysfs.  You say it doesn't matter,
> but if someone creates a script to check all of their stats via
> sysfs

Well, it matters if it actually takes a long time, but I don't really
care of it is O(n^2) and still only adding a few additional 100us's..

> > and nothing should use sysfs except
> > debugging.
> 
> Nobody manually checking on numbers themselves will use netlink.  And if
> the stats are there, people will check them.  You can't depend on this
> being used for debug access only.  If I recall correctly, ibstatus uses
> all sysfs entries, and people would easily think that using it uses the
> "preferred" method.  So, like I said, if it's there, it *will* get used,
> and not just for debug,

Why would people manually use sysfs? A netlink interface would be
accompanied by a tool. I don't schlep around in sysfs for netdev, I
use 'ip -s link show'

> > Caching is going to detrimental to apps that sync stats with external
> > time. (which is almost every real-world app)
> 
> That's problematic with or without caching as the stats don't have a
> timestamp, so scheduling delays could easily make the stats that you
> get

One can avoid scheduling delays with the right SCHED_ policy, how do
you avoid timing jitter from caching?

Jason
--
To unsubscribe from this list: send the line "unsubscribe linux-rdma" in
the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html