Provided by: xen-utils-common_4.17.3+10-g091466ba55-1.1ubuntu3_amd64 
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
<https://wiki.xenproject.org/wiki/Xen_on_NUMA_Machines> 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 <https://wiki.xenproject.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.
4.17.4-pre 2024-04-01 xl-numa-placement(7)