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[Xen-changelog] [xen master] docs: add pod variant of xl-numa-placement

Date: Sat, 05 Aug 2017 04:27:45 +0000

Delivery-date: Sat, 05 Aug 2017 04:27:50 +0000

List-id: "Change log for Mercurial \(receive only\)" <xen-changelog.lists.xen.org>

commit aa4eb460bcf77ea87b9209bb136efc8142a1a512 Author: Olaf Hering <olaf@xxxxxxxxx> AuthorDate: Wed Jul 26 16:39:50 2017 +0200 Commit: Wei Liu <wei.liu2@xxxxxxxxxx> CommitDate: Fri Jul 28 17:49:47 2017 +0100 docs: add pod variant of xl-numa-placement Convert source for xl-numa-placement.7 from markdown to pod. This removes the buildtime requirement for pandoc, and subsequently the need for ghc, in the chain for BuildRequires of xen.rpm. Signed-off-by: Olaf Hering <olaf@xxxxxxxxx> Reviewed-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx> Acked-by: Wei Liu <wei.liu2@xxxxxxxxxx> --- docs/man/xl-numa-placement.markdown.7 | 239 --------------------------- docs/man/xl-numa-placement.pod.7 | 293 ++++++++++++++++++++++++++++++++++ 2 files changed, 293 insertions(+), 239 deletions(-) diff --git a/docs/man/xl-numa-placement.markdown.7 b/docs/man/xl-numa-placement.markdown.7 deleted file mode 100644 index f863492..0000000 --- a/docs/man/xl-numa-placement.markdown.7 +++ /dev/null @@ -1,239 +0,0 @@ -# Guest Automatic NUMA Placement in libxl and xl # - -## 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][numa_intro] -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][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. - -[numa_intro]: http://wiki.xen.org/wiki/Xen_NUMA_Introduction -[cpupools_howto]: http://wiki.xen.org/wiki/Cpupools_Howto diff --git a/docs/man/xl-numa-placement.pod.7 b/docs/man/xl-numa-placement.pod.7 new file mode 100644 index 0000000..54a4441 --- /dev/null +++ b/docs/man/xl-numa-placement.pod.7 @@ -0,0 +1,293 @@ +=encoding utf8 + +=head1 NAME + +Guest Automatic NUMA Placement in libxl and xl + +=head1 DESCRIPTION + +=head2 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 L<Xen NUMA Introduction|http://wiki.xen.org/wiki/Xen_NUMA_Introduction> +page on the Wiki. + + +=head2 Xen and NUMA machines: the concept of I<node-affinity> + +The Xen hypervisor deals with NUMA machines throughout the concept of +I<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. + + +=head2 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 I<cpupool> with the "pool=" config option (as documented in our +L<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. + + +=head2 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: + +=over + +=item * + +a vCPU I<is pinned> to some pCPUs and I<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. + + +=item * + +a vCPU I<has> its own soft affinity and I<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; + + +=item * + +a vCPU I<has> its own vCPU soft affinity and I<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. + + +=back + + +=head2 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. + + +=head2 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: + +=over + +=item * + +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; + + +=item * + +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. + + +=back + + +=head2 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: + +=over + +=item * + +candidates involving fewer nodes are considered better. In case +two (or more) candidates span the same number of nodes, + + +=item * + +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, + + +=item * + +the candidate with with the greatest amount of free memory is +considered to be the best one. + + +=back + +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. + + +=head2 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 C<numa_placement> build info key to C<false> +(it is C<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 C<numa_placement> is set to C<true>, the domain's vCPUs must +not be pinned (i.e., C<<< domain_build_info->cpumap >>> must have all its +bits set, as it is by default), or domain creation will fail with +C<ERROR_INVAL>. + +Starting from Xen 4.3, in case automatic placement happens (and is +successful), it will affect the domain's node-affinity and I<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. + + +=head2 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. + + +=head2 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 I<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. + + +=head2 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. -- generated by git-patchbot for /home/xen/git/xen.git#master _______________________________________________ Xen-changelog mailing list Xen-changelog@xxxxxxxxxxxxx https://lists.xenproject.org/xen-changelog

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