Re: [RFC PATCH 5/5] SUNRPC: Reduce the priority of the xprtiod queue

[Chuck Lever <chuck.lever@oracle.com>]

> On May 28, 2019, at 3:52 PM, Chuck Lever <chuck.lever@oracle.com> wrote:
> 
> 
> 
>> On May 28, 2019, at 3:33 PM, Trond Myklebust <trondmy@hammerspace.com> wrote:
>> 
>> On Tue, 2019-05-28 at 15:03 -0400, Chuck Lever wrote:
>>> Following up on this. Now with even more data!
>>> 
>>>> On May 6, 2019, at 4:41 PM, Chuck Lever <chuck.lever@oracle.com>
>>>> wrote:
>>>> 
>>>> 
>>>>> On May 3, 2019, at 7:18 AM, Trond Myklebust <trondmy@gmail.com>
>>>>> wrote:
>>>>> 
>>>>> Allow more time for softirqd
>>>> 
>>>> Have you thought about performance tests for this one?
>>> 
>>> I tested this series on my 12-core two-socket client using a variety
>>> of tests including iozone, fio, and fstests. The network under test
>>> is 56Gb InfiniBand (TCP uses IPoIB). I tested both TCP and RDMA.
>>> 
>>> With lock debugging and memory leak testing enabled, I did not see
>>> any functional regressions or new leaks or crashes. Thus IMO this
>>> series is "safe to apply."
>>> 
>>> With TCP, I saw no change in performance between a "stock" kernel
>>> and one with all five patches in this series applied, as, IIRC,
>>> you predicted.
>>> 
>>> The following discussion is based on testing with NFS/RDMA.
>>> 
>>> With RDMA, I saw an improvement of 5-10% in IOPS rate between the
>>> "stock" kernel and a kernel with the first four patches applied. When
>>> the fifth patch is applied, I saw IOPS throughput significantly worse
>>> than "stock" -- like 20% worse.
>>> 
>>> I also studied average RPC execution time (the "execute" metric) with
>>> the "stock" kernel, the one with four patches applied, and with the
>>> one where all five are applied. The workload is 100% 4KB READs with
>>> an iodepth of 1024 in order to saturate the transmit queue.
>>> 
>>> With four patches, the execute time is about 2.5 msec faster (average
>>> execution time is around 75 msec due to the large backlog this test
>>> generates). With five patches, it's slower than "stock" by 12 msec.
>>> 
>>> I also saw a 30 usec improvement in the average latency of
>>> xprt_complete_rqst with the four patch series.
>>> 
>>> As far as I can tell, the benefit of this series comes mostly from
>>> the third patch, which changes spin_lock_bh(&xprt->transport_lock) to
>>> spin_lock(&xprt->transport_lock). When the xprtiod work queue is
>>> lowered in priority in 5/5, that benefit vanishes.
>>> 
>>> I am still confused about why 5/5 is needed. I did not see any soft
>>> lockups without this patch applied when using RDMA. Is the issue
>>> with xprtsock's use of xprtiod for handling incoming TCP receives?
>>> 
>>> I still have some things I'd like to look at. One thing I haven't
>>> yet tried is looking at lock_stat, which would confirm or refute
>>> my theory that this is all about the transport_lock, for instance.
>>> 
>> 
>> OK. I can drop 5/5.
>> 
>> The issue there was not about soft lockups. However since we were
>> previously running most soft irqs as part of spin_unlock_bh(), the
>> question was whether or not we would see more of them needing to move
>> to softirqd. As far as I can see, your answer to that question is 'no'
>> (at least for your system).
> 
> The top contended lock now is the work queue lock. I believe that's a
> full irqsave lock. Someone should try testing on a single core system.
> 
> I also plan to try this series on my mlx5_en system. The mlx5 Ethernet
> driver does a lot more work in soft IRQ than mlx4/IB does.

I tested with CX-5 RoCE on 100GbE. I don't see any obvious signs of
soft IRQ starvation. With 8 threads on a 4-core client, I was able to
push the 4KB random read fio workload past 300KIOPS.

--
Chuck Lever