Configured memory access latency and bandwidth not taking effect

Configured memory access latency and bandwidth not taking effect

[Thrivikraman V]

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I was trying to configure latencies and bandwidths between nodes in a NUMA
emulation using QEMU 5.0.0.

Host: Ubuntu 20.04 64 bit

Guest: Ubuntu 18.04 64 bit



The machine configured has 2 nodes. Each node has 2 CPUs and has been
allocated 3GB of memory. The memory access latencies and bandwidths for a
local access (i.e from initiator 0 to target 0, and from initiator 1 to
target 1) are set as 40ns and 10GB/s respectively. The memory access
latencies and bandwidths for a remote access (i.e from initiator 1 to
target 0, and from initiator 0 to target 1) are set as 80ns and 5GB/s
respectively.

The command-line launch is as follows.

sudo x86_64-softmmu/qemu-system-x86_64  \

-machine hmat=on \

-boot c \

-enable-kvm \

-m 6G,slots=2,maxmem=7G \

-object memory-backend-ram,size=3G,id=m0 \

-object memory-backend-ram,size=3G,id=m1 \

-numa node,nodeid=0,memdev=m0 \

-numa node,nodeid=1,memdev=m1 \

-smp 4,sockets=4,maxcpus=4  \

-numa cpu,node-id=0,socket-id=0 \

-numa cpu,node-id=0,socket-id=1 \

-numa cpu,node-id=1,socket-id=2 \

-numa cpu,node-id=1,socket-id=3 \

-numa dist,src=0,dst=1,val=20 \

-net nic \

-net user \

-hda testing.img \

-numa
hmat-lb,initiator=0,target=0,hierarchy=memory,data-type=access-latency,latency=40
\

-numa
hmat-lb,initiator=0,target=0,hierarchy=memory,data-type=access-bandwidth,bandwidth=10G
\

-numa
hmat-lb,initiator=0,target=1,hierarchy=memory,data-type=access-latency,latency=80
\

-numa
hmat-lb,initiator=0,target=1,hierarchy=memory,data-type=access-bandwidth,bandwidth=5G
\

-numa
hmat-lb,initiator=1,target=0,hierarchy=memory,data-type=access-latency,latency=80
\

-numa
hmat-lb,initiator=1,target=0,hierarchy=memory,data-type=access-bandwidth,bandwidth=5G
\

-numa
hmat-lb,initiator=1,target=1,hierarchy=memory,data-type=access-latency,latency=40
\

-numa
hmat-lb,initiator=1,target=1,hierarchy=memory,data-type=access-bandwidth,bandwidth=10G
\

Then the latencies and bandwidths between the nodes were tested using the Intel
Memory Latency Checker v3.9
<https://software.intel.com/content/www/us/en/develop/articles/intelr-memory-latency-checker.html>.
But the obtained results did not match the configuration. The following are
the results obtained.

Latency_matrix with idle latencies (in ns)

Numa node

0 1

0 36.2 36.4

1 34.9 35.4

Bandwidth_matrix with memory bandwidths (in MB/s)

Numa node

0 1

0 15167.1 15308.9

1 15226.0 15234.0

A test was also conducted with the tool “lat_mem_rd” from lmbench
<http://manpages.ubuntu.com/manpages/xenial/lmbench.8.html> to measure the
memory read latencies. This also gave results that did not match the config.

Any information on why the config latency and bandwidth values are not
applied, would be appreciated.

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