Re: [PATCH] nvme-pci: do not set the NUMA node of device if it has none

[Pratyush Yadav]

On Wed, Jul 26 2023, Christoph Hellwig wrote:

Hi all,

> On Wed, Jul 26, 2023 at 10:58:36AM +0300, Sagi Grimberg wrote:
>>>> For example, AWS EC2's i3.16xlarge instance does not expose NUMA
>>>> information for the NVMe devices. This means all NVMe devices have
>>>> NUMA_NO_NODE by default. Without this patch, random 4k read performance
>>>> measured via fio on CPUs from node 1 (around 165k IOPS) is almost 50%
>>>> less than CPUs from node 0 (around 315k IOPS). With this patch, CPUs on
>>>> both nodes get similar performance (around 315k IOPS).
>>>
>>> irqbalance doesn't work with this driver though: the interrupts are
>>> managed by the kernel. Is there some other reason to explain the perf
>>> difference?

Hmm, I did not know that. I have not gone and looked at the code but I
think the same reasoning should hold, just with s/irqbalance/kernel. If
the kernel IRQ balancer sees the device is on node 0, it would deliver
its interrupts to CPUs on node 0.

In my tests I can see that the interrupts for NVME queues are sent only
to CPUs from node 0 without this patch. With this patch CPUs from both
nodes get the interrupts.

>>
>> Maybe its because the numa_node goes to the tagset which allocates
>> stuff based on that numa-node ?
>
> Yeah, the only explanation I could come up with is that without this
> the allocations gets spread, and that somehow helps.  All of this
> is a little obscure, but so is the NVMe practice of setting the node id
> to first_memory_node, which no other driver does.  I'd really like to
> understand what's going on here first.  After that this patch probably
> is the right thing, I'd just like to understand why.

See above for my conjecture on why this happens.

More specifically, I discovered this when running an application pinned
to a node 1 CPU reading from an NVME device. I noticed it was performing
worse than when it was pinned to node 0.

If the process is free to move around it might not see such a large
performance difference since it could move to a node 0 CPU. But if it is
pinned to a CPU in node 1 then the interrupt will always hit a node 0
CPU and create higher latency for the reads.

I have a simple fio test that can reproduce this. Save this [1] as 
fio.txt and then run numactl --cpunodebind 1 fio ./fio.txt. You can run
it on any host with an NVME device that has no NUMA node. I have tested
this on AWS EC2's i3.16xlarge instance type.

[1]
    [global]
    ioengine=libaio
    filename=/dev/nvme0n1
    group_reporting=1
    direct=1
    verify=0
    norandommap=0
    size=10%
    time_based=1
    runtime=30
    ramp_time=0
    randrepeat=0
    log_max_value=1
    unified_rw_reporting=1
    percentile_list=50:99:99.9:99.99:99.999
    bwavgtime=10000

    [4k_randread_qd16_4w]
    stonewall
    bs=4k
    rw=randread
    iodepth=32
    numjobs=1

-- 
Regards,
Pratyush Yadav



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