Let’s say you’ve got a computer with a single processor and you want more computational power. A good option is to add more processors since they can mostly use the same technologies already present in your computer. Boom. Now you’ve got a symmetric multiprocessing (SMP) machine.

But pretty quickly you’re going to run into one of the drawbacks of SMP machines with a single main memory: bus contention. When data requests cannot be serviced from a processor’s cache, you get multiple processors trying to access memory via a single bus. The more processors you have, the worse the contention gets.

To fix this, you might decide to attach separate memory to each processor and call it local memory. Memory attached to remote processors becomes remote memory. Now each processor can access its own local memory using a separate memory bus. This is what many manufacturers started doing in the early 1990’s, and they called it Non-uniform Memory Access (NUMA), and named each group of processor, local memory, and I/O buses a NUMA node.

NUMA systems improve the performance of most multiprocessor workloads, but it’s rare for any workload to be perfectly confined to a single NUMA node. All kinds of things can happen that result in remote memory accesses, such as a task being migrated to a new NUMA node or running out of free local memory.

This might sound like the pre-1990 memory bus contention problem all over again, but the situation is actually worse now. Accessing remote memory takes much longer than accessing local memory, and not all remote memory has the same access latency. Depending on how the memory architecture is configured, NUMA nodes can be multiple hops away with each hop adding more latency.

So when a task exhausts all of local memory and the Linux kernel needs to grab free block from a remote node it has a decision to make: which remote node has enough free memory and the lowest access latency?

To figure that out, the kernel needs to know the access latency between any two NUMA nodes. And that’s something the firmware describes with the ACPI System Locality Distance Information Table (SLIT).

# What are SLIT tables?

System firmware provides ACPI SLIT tables (described in the section 5.2.17 of the ACPI 6.1 specification) that describe the relative differences in access latency between NUMA nodes. It’s basically a matrix that Linux reads on boot to build a map of NUMA memory latencies, and you can view it multiple ways: with numactl, the node/nodeX/distance sysfs file, or by dumping the ACPI tables directly with acpidump.

Here is the data from my NUMA test machine. It only has two NUMA nodes so it doesn’t give a good sense of how the node distances can vary, but at least we’ve got much less data to read.

The distances (memory latency) between a node and itself is normalised to 10, and every other distance is scaled relative to that 10 base value. So in the above example, the distance between NUMA node 0 and NUMA node 1 is 2.1 and the table shows a value of 21. In other words, if NUMA node 0 accesses memory on NUMA node 1 or vice versa, access latency will be 2.1x more than for local memory.

At least, that’s what the ACPI tables claim.

# How accurate are the SLIT table values?

It’s a perennial problem with ACPI tables that the values stored are not always accurate, or even necessarily true. Suspecting that might be the case with SLIT tables, I decided to run Intel’s Memory Latency Checker tool to measure the actual main memory read latency between NUMA nodes on my test machine.

Measuring memory latency is notoriously difficult on modern machines because of hardware prefetchers. Fortunately, MLC disables prefetchers using the Linux msr module but that also means you need to run it as root.

A bit of quick maths shows that these latency measurements do not match with the SLIT values. In fact, the relative distance between node 0 and 1 is closer to 1.5.

# OK, but who cares?

Remember when I said that NUMA node distances were used by the kernel? One of the places they’re used is figuring out whether to load balance tasks between nodes. Moving tasks across NUMA nodes is costly, so the kernel tries to avoid it whenever it can.

How costly is too costly? The current magic value used inside Linux kernel is 30 – if the NUMA node distance between two nodes is more than 30, the Linux kernel scheduler will try not to migrate tasks between them.

Of course, if the ACPI tables are wrong, and claim a distance of 30 or more but in reality it’s less, you’re unnecessarily impacting performance because you’ll likely see one extremely busy NUMA node while over nodes sit idle. And the scheduler will refuse to migrate tasks to the idle nodes until the active load balancer kicks in.

This patch fixes this exact bug for AMD EYPC machines which report a distance of 32 between some nodes (I measured the memory latency with mlc and it’s definitely less than 3.2x). With the patch applied you get a nice 20-30% improvement to CPU-intensive benchmarks because the scheduler balances tasks across the entire system.

Ugh. Firmware.