On 2021-01-18 16:18, Chuck Lever wrote:
On Jan 12, 2021, at 9:38 AM, Will Deacon <will@xxxxxxxxxx> wrote:
[Expanding cc list to include DMA-IOMMU and intel IOMMU folks]
On Fri, Jan 08, 2021 at 04:18:36PM -0500, Chuck Lever wrote:
Hi-
[ Please cc: me on replies, I'm not currently subscribed to
iommu@lists ].
I'm running NFS performance tests on InfiniBand using CX-3 Pro cards
at 56Gb/s. The test is iozone on an NFSv3/RDMA mount:
/home/cel/bin/iozone -M -+u -i0 -i1 -s1g -r256k -t12 -I
For those not familiar with the way storage protocols use RDMA, The
initiator/client sets up memory regions and the target/server uses
RDMA Read and Write to move data out of and into those regions. The
initiator/client uses only RDMA memory registration and invalidation
operations, and the target/server uses RDMA Read and Write.
My NFS client is a two-socket 12-core x86_64 system with its I/O MMU
enabled using the kernel command line options "intel_iommu=on
iommu=strict".
Recently I've noticed a significant (25-30%) loss in NFS throughput.
I was able to bisect on my client to the following commits.
Here's 65f746e8285f ("iommu: Add quirk for Intel graphic devices in
map_sg"). This is about normal for this test.
Children see throughput for 12 initial writers = 4732581.09 kB/sec
Parent sees throughput for 12 initial writers = 4646810.21 kB/sec
Min throughput per process = 387764.34 kB/sec
Max throughput per process = 399655.47 kB/sec
Avg throughput per process = 394381.76 kB/sec
Min xfer = 1017344.00 kB
CPU Utilization: Wall time 2.671 CPU time 1.974 CPU utilization 73.89 %
Children see throughput for 12 rewriters = 4837741.94 kB/sec
Parent sees throughput for 12 rewriters = 4833509.35 kB/sec
Min throughput per process = 398983.72 kB/sec
Max throughput per process = 406199.66 kB/sec
Avg throughput per process = 403145.16 kB/sec
Min xfer = 1030656.00 kB
CPU utilization: Wall time 2.584 CPU time 1.959 CPU utilization 75.82 %
Children see throughput for 12 readers = 5921370.94 kB/sec
Parent sees throughput for 12 readers = 5914106.69 kB/sec
Min throughput per process = 491812.38 kB/sec
Max throughput per process = 494777.28 kB/sec
Avg throughput per process = 493447.58 kB/sec
Min xfer = 1042688.00 kB
CPU utilization: Wall time 2.122 CPU time 1.968 CPU utilization 92.75 %
Children see throughput for 12 re-readers = 5947985.69 kB/sec
Parent sees throughput for 12 re-readers = 5941348.51 kB/sec
Min throughput per process = 492805.81 kB/sec
Max throughput per process = 497280.19 kB/sec
Avg throughput per process = 495665.47 kB/sec
Min xfer = 1039360.00 kB
CPU utilization: Wall time 2.111 CPU time 1.968 CPU utilization 93.22 %
Here's c062db039f40 ("iommu/vt-d: Update domain geometry in
iommu_ops.at(de)tach_dev"). It's losing some steam here.
Children see throughput for 12 initial writers = 4342419.12 kB/sec
Parent sees throughput for 12 initial writers = 4310612.79 kB/sec
Min throughput per process = 359299.06 kB/sec
Max throughput per process = 363866.16 kB/sec
Avg throughput per process = 361868.26 kB/sec
Min xfer = 1035520.00 kB
CPU Utilization: Wall time 2.902 CPU time 1.951 CPU utilization 67.22 %
Children see throughput for 12 rewriters = 4408576.66 kB/sec
Parent sees throughput for 12 rewriters = 4404280.87 kB/sec
Min throughput per process = 364553.88 kB/sec
Max throughput per process = 370029.28 kB/sec
Avg throughput per process = 367381.39 kB/sec
Min xfer = 1033216.00 kB
CPU utilization: Wall time 2.836 CPU time 1.956 CPU utilization 68.97 %
Children see throughput for 12 readers = 5406879.47 kB/sec
Parent sees throughput for 12 readers = 5401862.78 kB/sec
Min throughput per process = 449583.03 kB/sec
Max throughput per process = 451761.69 kB/sec
Avg throughput per process = 450573.29 kB/sec
Min xfer = 1044224.00 kB
CPU utilization: Wall time 2.323 CPU time 1.977 CPU utilization 85.12 %
Children see throughput for 12 re-readers = 5410601.12 kB/sec
Parent sees throughput for 12 re-readers = 5403504.40 kB/sec
Min throughput per process = 449918.12 kB/sec
Max throughput per process = 452489.28 kB/sec
Avg throughput per process = 450883.43 kB/sec
Min xfer = 1043456.00 kB
CPU utilization: Wall time 2.321 CPU time 1.978 CPU utilization 85.21 %
And here's c588072bba6b ("iommu/vt-d: Convert intel iommu driver to
the iommu ops"). Significant throughput loss.
Children see throughput for 12 initial writers = 3812036.91 kB/sec
Parent sees throughput for 12 initial writers = 3753683.40 kB/sec
Min throughput per process = 313672.25 kB/sec
Max throughput per process = 321719.44 kB/sec
Avg throughput per process = 317669.74 kB/sec
Min xfer = 1022464.00 kB
CPU Utilization: Wall time 3.309 CPU time 1.986 CPU utilization 60.02 %
Children see throughput for 12 rewriters = 3786831.94 kB/sec
Parent sees throughput for 12 rewriters = 3783205.58 kB/sec
Min throughput per process = 313654.44 kB/sec
Max throughput per process = 317844.50 kB/sec
Avg throughput per process = 315569.33 kB/sec
Min xfer = 1035520.00 kB
CPU utilization: Wall time 3.302 CPU time 1.945 CPU utilization 58.90 %
Children see throughput for 12 readers = 4265828.28 kB/sec
Parent sees throughput for 12 readers = 4261844.88 kB/sec
Min throughput per process = 352305.00 kB/sec
Max throughput per process = 357726.22 kB/sec
Avg throughput per process = 355485.69 kB/sec
Min xfer = 1032960.00 kB
CPU utilization: Wall time 2.934 CPU time 1.942 CPU utilization 66.20 %
Children see throughput for 12 re-readers = 4220651.19 kB/sec
Parent sees throughput for 12 re-readers = 4216096.04 kB/sec
Min throughput per process = 348677.16 kB/sec
Max throughput per process = 353467.44 kB/sec
Avg throughput per process = 351720.93 kB/sec
Min xfer = 1035264.00 kB
CPU utilization: Wall time 2.969 CPU time 1.952 CPU utilization 65.74 %
The regression appears to be 100% reproducible.
Any thoughts?
How about some tools to try or debugging advice? I don't know where to start.
I'm not familiar enough with VT-D internals or Infiniband to have a clue
why the middle commit makes any difference (the calculation itself is
not on a fast path, so AFAICS the worst it could do is change your
maximum DMA address size from 48/57 bits to 47/56, and that seems
relatively benign).
With the last commit, though, at least part of it is likely to be the
unfortunate inevitable overhead of the internal indirection through the
IOMMU API. There's a coincidental performance-related thread where we've
already started pondering some ideas in that area[1] (note that Intel is
the last one to the party here; AMD has been using this path for a
while, and it's all that arm64 systems have ever known). I'm not sure if
there's any difference in the strict invalidation behaviour between the
IOMMU API calls and the old intel_dma_ops, but I suppose that might be
worth quickly double-checking as well. I guess the main thing would be
to do some profiling to see where time is being spent in iommu-dma and
intel-iommu vs. just different parts of intel-iommu before, and whether
anything in particular stands out beyond the extra call overhead
currently incurred by iommu_{map,unmap}.
I'm mildly puzzled by your "CPU time" metric remaining more or less
constant and the utilisation jumping up and down though, or is that only
counting time in the userspace workload such that spending more time
busy in the kernel skews it?
Robin.
[1]
https://lore.kernel.org/linux-iommu/20210112163307.GA1199965@xxxxxxxxxxxxx/
