Mark Hahn wrote:
I suspect that the main problem is that this patch is not a mainstream
kernel feature that will gain multiple uses, but rather provides
support for a specific vendor middleware product used by that
vendor and a few closely allied vendors. If it were smaller or
less intrusive, such as a driver, this would not be a big problem.
That's not the case.
yes, that's the crux. CKRM is all about resolving conflicting resource
demands in a multi-user, multi-server, multi-purpose machine. this is a
huge undertaking, and I'd argue that it's completely inappropriate for
*most* servers. that is, computers are generally so damn cheap that
the clear trend is towards dedicating a machine to a specific purpose,
rather than running eg, shell/MUA/MTA/FS/DB/etc all on a single machine.
The argument about scale-up vs. scale-out is nowhere close to being
resolved. To argue that any support for performance partitioning (which
CKRM does) is in support of a lost cause is premature to say the least.
this is *directly* in conflict with certain prominent products, such as
the Altix and various less-prominent Linux-based mainframes. they're all
about partitioning/virtualization - the big-iron aesthetic of splitting up
a single machine. note that it's not just about "big", since cluster-based
approaches can clearly scale far past big-iron, and are in effect statically
partitioned. yes, buying a hideously expensive single box, and then chopping
it into little pieces is more than a little bizarre, and is mainly based
on a couple assumptions:
- that clusters are hard. really, they aren't. they are not
necessarily higher-maintenance, can be far more robust, usually
do cost less. just about the only bad thing about clusters is
that they tend to be somewhat larger in size.
- that partitioning actually makes sense. the appeal is that if
you have a partition to yourself, you can only hurt yourself.
but it also follows that burstiness in resource demand cannot be
overlapped without either constantly tuning the partitions or
infringing on the guarantee.
"constantly tuning the partitions" is effectively whats done by workload
managers. But our earlier presentations and papers have made the case
that this is not the only utility for performance isolation - simple
needs like isolating one user from another on a general purpose server
is also a need that cannot be met by any existing or proposed Linux
kernel mechanisms today.
If partitioning made so little sense and the case for clusters was that
obvious, one would be hard put to explain why server consolidation is
being actively pursued by so many firms, Solaris is bothering with
coming up with Containers and Xen/VMWare getting all this attention.
I don't think the concept of partitioning can be dismissed so easily.
Of course, it must be noted that CKRM only provides performance
isolation not fault isolation. But there is a need for that. Whether
Linux chooses to let this need influence its design is another matter
(which I hope we'll also discuss besides the implementation issues).
CKRM is one of those things that could be done to Linux, and will benefit a
few, but which will almost certainly hurt *most* of the community.
let me say that the CKRM design is actually quite good. the issue is whether
the extensive hooks it requires can be done (at all) in a way which does
not disporportionately hurt maintainability or efficiency.
If there are suggestions on implementing this better, it'll certainly be
very welcome.
CKRM requires hooks into every resource-allocation decision fastpath:
- if CKRM is not CONFIG, the only overhead is software maintenance.
- if CKRM is CONFIG but not loaded, the overhead is a pointer check.
- if CKRM is CONFIG and loaded, the overhead is a pointer check
and a nontrivial callback.
but really, this is only for CKRM-enforced limits. CKRM really wants to
change behavior in a more "weighted" way, not just causing an
allocation/fork/packet to fail. a really meaningful CKRM needs to
be tightly integrated into each resource manager - effecting each scheduler
(process, memory, IO, net). I don't really see how full-on CKRM can be
compiled out, unless these schedulers are made fully pluggable.
This is a valid point for the CPU, memory and network controllers (I/O
can be made pluggable quite easily). For the CPU controller in SuSE, the
CKRM CPU controller can be turned on and off dynamically at runtime.
Exploring a similar option for memory and network (incurring only a
pointer check) could be explored. Keeping the overhead close to zero for
kernel users not interested in CKRM is certainly one of our objectives.
finally, I observe that pluggable, class-based resource _limits_ could
probably be done without callbacks and potentially with low overhead.
but mere limits doesn't meet CKRM's goal of flexible, wide-spread resource
partitioning within a large, shared machine.
True but only limits are not as useful for general workload management.
regards, mark hahn.
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