Re: [Patch][RFC] Disabling per-tgid stats on task exit in taskstats

Andrew Morton wrote:

On Thu, 29 Jun 2006 09:44:08 -0700
Paul Jackson [off-list ref] wrote:

 

quoted

quoted

You're probably correct on that model. However, it all depends on the actual
workload. Are people who actually have large-CPU (>256) systems actually
running fork()-heavy things like webservers on them, or are they running things
like database servers and computations, which tend to have persistent
processes?
     

It may well be mostly as you say - the large-CPU systems not running
the fork() heavy jobs.

Sooner or later, someone will want to run a fork()-heavy job on a
large-CPU system.  On a 1024 CPU system, it would apparently take
just 14 exits/sec/CPU to hit this bottleneck, if Jay's number of
14000 applied.

Chris Sturdivant's reply is reasonable -- we'll hit it sooner or later,
and deal with it then.

   

I agree, and I'm viewing this as blocking the taskstats merge.  Because if
this _is_ a problem then it's a big one because fixing it will be
intrusive, and might well involve userspace-visible changes.
 

First off, just a reminder that this is inherently a netlink flow 
control issue...which was being exacerbated
earlier by taskstats decision to send per-tgid data (no longer the case).

But I'd like to know whats our target here ? How many messages per 
second do we want to be able to be sent
and received without risking any loss of data ? Netlink will lose 
messages at a high enough rate so the design point
will need to be known a bit.

For statistics type usage of the genetlink/netlink, I would have thought 
that userspace, provided it is reliably informed
about the loss of data through ENOBUFS, could take measures to just 
account for the missing data and carry on ?

The only ways I can see of fixing the problem generally are to either

a) throw more CPU(s) at stats collection: allow userspace to register for
  "stats generated by CPU N", then run a stats collection daemon on each
  CPU or
 

b) make the kernel recognise when it's getting overloaded and switch to
  some degraded mode where it stops trying to send all the data to
  userspace - just send a summary, or a "we goofed" message or something.
 

One of the unused features of genetlink that's meant for high volume 
data output from the kernel is
the "dump" callback of a genetlink connection. Essentially kernel space 
keeps getting provided sk_buffs
to fill which the netlink layer then supplies to user space (over time I 
guess ?)

But whatever we do, there's going to be some limit so its useful to 
decide what the design point should be ?

Adding Jamal for his thoughts on netlink's flow control in the context 
of genetlink.

--Shailabh