NAME
xl-numa-placement - Guest Automatic NUMA Placement in libxl and xl
DESCRIPTION
Rationale
NUMA (which stands for Non-Uniform Memory Access) means that the memory
accessing times of a program running on a CPU depends on the relative
distance between that CPU and that memory. In fact, most of the NUMA
systems are built in such a way that each processor has its local
memory, on which it can operate very fast. On the other hand, getting
and storing data from and on remote memory (that is, memory local to
some other processor) is quite more complex and slow. On these machines,
a NUMA node is usually defined as a set of processor cores (typically a
physical CPU package) and the memory directly attached to the set of
cores.
NUMA awareness becomes very important as soon as many domains start
running memory-intensive workloads on a shared host. In fact, the cost
of accessing non node-local memory locations is very high, and the
performance degradation is likely to be noticeable.
For more information, have a look at the Xen NUMA Introduction
<http://wiki.xen.org/wiki/Xen_NUMA_Introduction> page on the Wiki.
Xen and NUMA machines: the concept of *node-affinity*
The Xen hypervisor deals with NUMA machines throughout the concept of
*node-affinity*. The node-affinity of a domain is the set of NUMA nodes
of the host where the memory for the domain is being allocated (mostly,
at domain creation time). This is, at least in principle, different and
unrelated with the vCPU (hard and soft, see below) scheduling affinity,
which instead is the set of pCPUs where the vCPU is allowed (or prefers)
to run.
Of course, despite the fact that they belong to and affect different
subsystems, the domain node-affinity and the vCPUs affinity are not
completely independent. In fact, if the domain node-affinity is not
explicitly specified by the user, via the proper libxl calls or xl
config item, it will be computed basing on the vCPUs' scheduling
affinity.
Notice that, even if the node affinity of a domain may change on-line,
it is very important to "place" the domain correctly when it is fist
created, as the most of its memory is allocated at that time and can not
(for now) be moved easily.
Placing via pinning and cpupools
The simplest way of placing a domain on a NUMA node is setting the hard
scheduling affinity of the domain's vCPUs to the pCPUs of the node. This
also goes under the name of vCPU pinning, and can be done through the
"cpus=" option in the config file (more about this below). Another
option is to pool together the pCPUs spanning the node and put the
domain in such a *cpupool* with the "pool=" config option (as documented
in our Wiki <http://wiki.xen.org/wiki/Cpupools_Howto>).
In both the above cases, the domain will not be able to execute outside
the specified set of pCPUs for any reasons, even if all those pCPUs are
busy doing something else while there are others, idle, pCPUs.
So, when doing this, local memory accesses are 100% guaranteed, but that
may come at he cost of some load imbalances.
NUMA aware scheduling
If using the credit1 scheduler, and starting from Xen 4.3, the scheduler
itself always tries to run the domain's vCPUs on one of the nodes in its
node-affinity. Only if that turns out to be impossible, it will just
pick any free pCPU. Locality of access is less guaranteed than in the
pinning case, but that comes along with better chances to exploit all
the host resources (e.g., the pCPUs).
Starting from Xen 4.5, credit1 supports two forms of affinity: hard and
soft, both on a per-vCPU basis. This means each vCPU can have its own
soft affinity, stating where such vCPU prefers to execute on. This is
less strict than what it (also starting from 4.5) is called hard
affinity, as the vCPU can potentially run everywhere, it just prefers
some pCPUs rather than others. In Xen 4.5, therefore, NUMA-aware
scheduling is achieved by matching the soft affinity of the vCPUs of a
domain with its node-affinity.
In fact, as it was for 4.3, if all the pCPUs in a vCPU's soft affinity
are busy, it is possible for the domain to run outside from there. The
idea is that slower execution (due to remote memory accesses) is still
better than no execution at all (as it would happen with pinning). For
this reason, NUMA aware scheduling has the potential of bringing
substantial performances benefits, although this will depend on the
workload.
Notice that, for each vCPU, the following three scenarios are possbile:
* a vCPU *is pinned* to some pCPUs and *does not have* any soft
affinity In this case, the vCPU is always scheduled on one of the
pCPUs to which it is pinned, without any specific peference among
them.
* a vCPU *has* its own soft affinity and *is not* pinned to any
particular pCPU. In this case, the vCPU can run on every pCPU.
Nevertheless, the scheduler will try to have it running on one of
the pCPUs in its soft affinity;
* a vCPU *has* its own vCPU soft affinity and *is also* pinned to some
pCPUs. In this case, the vCPU is always scheduled on one of the
pCPUs onto which it is pinned, with, among them, a preference for
the ones that also forms its soft affinity. In case pinning and soft
affinity form two disjoint sets of pCPUs, pinning "wins", and the
soft affinity is just ignored.
Guest placement in xl
If using xl for creating and managing guests, it is very easy to ask for
both manual or automatic placement of them across the host's NUMA nodes.
Note that xm/xend does a very similar thing, the only differences being
the details of the heuristics adopted for automatic placement (see
below), and the lack of support (in both xm/xend and the Xen versions
where that was the default toolstack) for NUMA aware scheduling.
Placing the guest manually
Thanks to the "cpus=" option, it is possible to specify where a domain
should be created and scheduled on, directly in its config file. This
affects NUMA placement and memory accesses as, in this case, the
hypervisor constructs the node-affinity of a VM basing right on its vCPU
pinning when it is created.
This is very simple and effective, but requires the user/system
administrator to explicitly specify the pinning for each and every
domain, or Xen won't be able to guarantee the locality for their memory
accesses.
That, of course, also mean the vCPUs of the domain will only be able to
execute on those same pCPUs.
It is is also possible to have a "cpus_soft=" option in the xl config
file, to specify the soft affinity for all the vCPUs of the domain. This
affects the NUMA placement in the following way:
* if only "cpus_soft=" is present, the VM's node-affinity will be
equal to the nodes to which the pCPUs in the soft affinity mask
belong;
* if both "cpus_soft=" and "cpus=" are present, the VM's node-affinity
will be equal to the nodes to which the pCPUs present both in hard
and soft affinity belong.
Placing the guest automatically
If neither "cpus=" nor "cpus_soft=" are present in the config file,
libxl tries to figure out on its own on which node(s) the domain could
fit best. If it finds one (some), the domain's node affinity get set to
there, and both memory allocations and NUMA aware scheduling (for the
credit scheduler and starting from Xen 4.3) will comply with it.
Starting from Xen 4.5, this also means that the mask resulting from this
"fitting" procedure will become the soft affinity of all the vCPUs of
the domain.
It is worthwhile noting that optimally fitting a set of VMs on the NUMA
nodes of an host is an incarnation of the Bin Packing Problem. In fact,
the various VMs with different memory sizes are the items to be packed,
and the host nodes are the bins. As such problem is known to be NP-hard,
we will be using some heuristics.
The first thing to do is find the nodes or the sets of nodes (from now
on referred to as 'candidates') that have enough free memory and enough
physical CPUs for accommodating the new domain. The idea is to find a
spot for the domain with at least as much free memory as it has
configured to have, and as much pCPUs as it has vCPUs. After that, the
actual decision on which candidate to pick happens accordingly to the
following heuristics:
* candidates involving fewer nodes are considered better. In case two
(or more) candidates span the same number of nodes,
* candidates with a smaller number of vCPUs runnable on them (due to
previous placement and/or plain vCPU pinning) are considered better.
In case the same number of vCPUs can run on two (or more)
candidates,
* the candidate with with the greatest amount of free memory is
considered to be the best one.
Giving preference to candidates with fewer nodes ensures better
performance for the guest, as it avoid spreading its memory among
different nodes. Favoring candidates with fewer vCPUs already runnable
there ensures a good balance of the overall host load. Finally, if more
candidates fulfil these criteria, prioritizing the nodes that have the
largest amounts of free memory helps keeping the memory fragmentation
small, and maximizes the probability of being able to put more domains
there.
Guest placement in libxl
xl achieves automatic NUMA placement because that is what libxl does by
default. No API is provided (yet) for modifying the behaviour of the
placement algorithm. However, if your program is calling libxl, it is
possible to set the "numa_placement" build info key to "false" (it is
"true" by default) with something like the below, to prevent any
placement from happening:
libxl_defbool_set(&domain_build_info->numa_placement, false);
Also, if "numa_placement" is set to "true", the domain's vCPUs must not
be pinned (i.e., "domain_build_info->cpumap" must have all its bits set,
as it is by default), or domain creation will fail with "ERROR_INVAL".
Starting from Xen 4.3, in case automatic placement happens (and is
successful), it will affect the domain's node-affinity and *not* its
vCPU pinning. Namely, the domain's vCPUs will not be pinned to any pCPU
on the host, but the memory from the domain will come from the selected
node(s) and the NUMA aware scheduling (if the credit scheduler is in
use) will try to keep the domain's vCPUs there as much as possible.
Besides than that, looking and/or tweaking the placement algorithm
search "Automatic NUMA placement" in libxl_internal.h.
Note this may change in future versions of Xen/libxl.
Xen < 4.5
The concept of vCPU soft affinity has been introduced for the first time
in Xen 4.5. In 4.3, it is the domain's node-affinity that drives the
NUMA-aware scheduler. The main difference is soft affinity is per-vCPU,
and so each vCPU can have its own mask of pCPUs, while node-affinity is
per-domain, that is the equivalent of having all the vCPUs with the same
soft affinity.
Xen < 4.3
As NUMA aware scheduling is a new feature of Xen 4.3, things are a
little bit different for earlier version of Xen. If no "cpus=" option is
specified and Xen 4.2 is in use, the automatic placement algorithm still
runs, but the results is used to *pin* the vCPUs of the domain to the
output node(s). This is consistent with what was happening with xm/xend.
On a version of Xen earlier than 4.2, there is not automatic placement
at all in xl or libxl, and hence no node-affinity, vCPU affinity or
pinning being introduced/modified.
Limitations
Analyzing various possible placement solutions is what makes the
algorithm flexible and quite effective. However, that also means it
won't scale well to systems with arbitrary number of nodes. For this
reason, automatic placement is disabled (with a warning) if it is
requested on a host with more than 16 NUMA nodes.
