Re: NFS over RDMA benchmark

[Tom Talpey <tom@talpey.com>]

On 4/30/2013 1:09 AM, Yan Burman wrote:
>
>
>> -----Original Message-----
>> From: J. Bruce Fields [mailto:bfields@fieldses.org]
>> Sent: Sunday, April 28, 2013 17:43
>> To: Yan Burman
>> Cc: Wendy Cheng; Atchley, Scott; Tom Tucker; linux-rdma@vger.kernel.org;
>> linux-nfs@vger.kernel.org; Or Gerlitz
>> Subject: Re: NFS over RDMA benchmark
>>
>> On Sun, Apr 28, 2013 at 06:28:16AM +0000, Yan Burman wrote:
>>>>>>>>>>> On Wed, Apr 17, 2013 at 7:36 AM, Yan Burman
>>>>>>>>>>> <yanb@mellanox.com>
>>>>>>>>>>>> I've been trying to do some benchmarks for NFS over
>>>>>>>>>>>> RDMA and I seem to
>>>>>>>>> only get about half of the bandwidth that the HW can give me.
>>>>>>>>>>>> My setup consists of 2 servers each with 16 cores,
>>>>>>>>>>>> 32Gb of memory, and
>>>>>>>>> Mellanox ConnectX3 QDR card over PCI-e gen3.
>>>>>>>>>>>> These servers are connected to a QDR IB switch. The
>>>>>>>>>>>> backing storage on
>>>>>>>>> the server is tmpfs mounted with noatime.
>>>>>>>>>>>> I am running kernel 3.5.7.
>>>>>>>>>>>>
>>>>>>>>>>>> When running ib_send_bw, I get 4.3-4.5 GB/sec for
>>>>>>>>>>>> block sizes 4-
>>>> 512K.
>>>>>>>>>>>> When I run fio over rdma mounted nfs, I get
>>>>>>>>>>>> 260-2200MB/sec for the
>>>>>>>>> same block sizes (4-512K). running over IPoIB-CM, I get
>>>>>>>>> 200-
>>>> 980MB/sec.
>> ...
>>>>>>>> I am trying to get maximum performance from a single server
>>>>>>>> - I used 2
>>>>>>> processes in fio test - more than 2 did not show any performance
>> boost.
>>>>>>>> I tried running fio from 2 different PCs on 2 different
>>>>>>>> files, but the sum of
>>>>>>> the two is more or less the same as running from single client PC.
>>>>>>>>
>
> I finally got up to 4.1GB/sec bandwidth with RDMA (ipoib-CM bandwidth is also way higher now).
> For some reason when I had intel IOMMU enabled, the performance dropped significantly.
> I now get up to ~95K IOPS and 4.1GB/sec bandwidth.

Excellent, but is that 95K IOPS a typo? At 4KB, that's less than 400MBps.

What is the client CPU percentage you see under this workload, and
how different are the NFS/RDMA and NFS/IPoIB overheads?

> Now I will take care of the issue that I am running only at 40Gbit/s instead of 56Gbit/s, but that is another unrelated problem (I suspect I have a cable issue).
>
> This is still strange, since ib_send_bw with intel iommu enabled did get up to 4.5GB/sec, so why did intel iommu affect only nfs code?

You'll need to do more profiling to track that down. I would suspect
that ib_send_bw is using some sort of direct hardware access, bypassing
the IOMMU management and possibly performing no dynamic memory registration.

The NFS/RDMA code goes via the standard kernel DMA API, and correctly
registers/deregisters memory on a per-i/o basis in order to provide
storage data integrity. Perhaps there are overheads in the IOMMU
management which can be addressed.