Re: [PATCH RFC net-next] net/smc: transition to RDMA core CQ pooling

Mahanta Jambigi
2026-02-13T11:23:48Z

On 09/02/26 1:23 pm, D. Wythe wrote:
> On Fri, Feb 06, 2026 at 04:58:23PM +0530, Mahanta Jambigi wrote:
>>
>>
>> On 02/02/26 3:18 pm, D. Wythe wrote:
>>> The current SMC-R implementation relies on global per-device CQs
>>> and manual polling within tasklets, which introduces severe
>>> scalability bottlenecks due to global lock contention and tasklet
>>> scheduling overhead, resulting in poor performance as concurrency
>>> increases.
>>>
>>> Refactor the completion handling to utilize the ib_cqe API and
>>> standard RDMA core CQ pooling. This transition provides several key
>>> advantages:
>>>
>>> 1. Multi-CQ: Shift from a single shared per-device CQ to multiple
>>> link-specific CQs via the CQ pool. This allows completion processing
>>> to be parallelized across multiple CPU cores, effectively eliminating
>>> the global CQ bottleneck.
>>>
>>> 2. Leverage DIM: Utilizing the standard CQ pool with IB_POLL_SOFTIRQ
>>> enables Dynamic Interrupt Moderation from the RDMA core, optimizing
>>> interrupt frequency and reducing CPU load under high pressure.
>>>
>>> 3. O(1) Context Retrieval: Replaces the expensive wr_id based lookup
>>> logic (e.g., smc_wr_tx_find_pending_index) with direct context retrieval
>>> using container_of() on the embedded ib_cqe.
>>>
>>> 4. Code Simplification: This refactoring results in a reduction of
>>> ~150 lines of code. It removes redundant sequence tracking, complex lookup
>>> helpers, and manual CQ management, significantly improving maintainability.
>>>
>>> Performance Test: redis-benchmark with max 32 connections per QP
>>> Data format: Requests Per Second (RPS), Percentage in brackets
>>> represents the gain/loss compared to TCP.
>>>
>>> | Clients | TCP | SMC (original) | SMC (cq_pool) |
>>> |---------|----------|---------------------|---------------------|
>>> | c = 1 | 24449 | 31172 (+27%) | 34039 (+39%) |
>>> | c = 2 | 46420 | 53216 (+14%) | 64391 (+38%) |
>>> | c = 16 | 159673 | 83668 (-48%) <-- | 216947 (+36%) |
>>> | c = 32 | 164956 | 97631 (-41%) <-- | 249376 (+51%) |
>>> | c = 64 | 166322 | 118192 (-29%) <-- | 249488 (+50%) |
>>> | c = 128 | 167700 | 121497 (-27%) <-- | 249480 (+48%) |
>>> | c = 256 | 175021 | 146109 (-16%) <-- | 240384 (+37%) |
>>> | c = 512 | 168987 | 101479 (-40%) <-- | 226634 (+34%) |
>>>
>>> The results demonstrate that this optimization effectively resolves the
>>> scalability bottleneck, with RPS increasing by over 110% at c=64
>>> compared to the original implementation.
>>
>> I applied your patch to the latest kernel(6.19-rc8) & saw below
>> Performance results:
>>
>> 1) In my evaluation, I ran several *uperf* based workloads using a
>> request/response (RR) pattern, and I observed performance *degradation*
>> ranging from *4%* to *59%*, depending on the specific read/write sizes
>> used. For example, with a TCP RR workload using 50 parallel clients
>> (nprocs=50) sending a 200‑byte request and reading a 1000‑byte response
>> over a 60‑second run, I measured approximately 59% degradation compared
>> to SMC‑R original performance.
>>
>
> The only setting I changed was net.smc.smcr_max_conns_per_lgr = 32, all
> other parameters were left at their default values. redis-benchmark is a
> classic Request/Response (RR) workload, which contradicts your test
> results. Since I'm unable to reproduce your results, it would be
> very helpful if you could share the specific test configuration for my
> analysis.

I used a simple client–server setup connected via 25 Gb/s RoCE_Express2
adapters on the same LAN(connection established via SMC-R v1). After
running the commands shown below, I observed a performance degradation
of up to 59%.

Server: smc_run uperf -s
Client: smc_run uperf -m rr1c-200x1000-50.xml

cat rr1c-200x1000-50.xml

<?xml version="1.0"?>
<profile name="TCP_RR">
<group nprocs="50">
<transaction iterations="1">
<flowop type="connect" options="remotehost=server_ip protocol=tcp
tcp_nodelay" />
</transaction>
<transaction duration="60">
<flowop type="write" options="size=200"/>
<flowop type="read" options="size=1000"/>
</transaction>
<transaction iterations="1">
<flowop type="disconnect" />
</transaction>
</group>
</profile>

I installed redis-server on the server machine & redis-benchmark on the
client machine & I was able to establish the SMC-R using below commands.
If you could help me with the exact commands you used to measure the
redis-benchmark performance, I can try the same on my setup.

Server: smc_run redis-server --port <port_num> --save "" --appendonly no
--protected-mode no --bind 0.0.0.0
Client: smc_run redis-benchmark -h <server_ip> -p <port_num> -n 10000 -c
50 -t ping_inline,ping_bulk -q