Re: [for-next PATCH] infiniband:cma: add a parameter for the packet lifetime

[Chao Leng <lengchao@huawei.com>]

On 2022/11/24 3:48, Jason Gunthorpe wrote:
> On Wed, Nov 23, 2022 at 10:13:48AM +0800, Chao Leng wrote:
>>
>>
>> On 2022/11/22 22:08, Jason Gunthorpe wrote:
>>> On Tue, Nov 22, 2022 at 05:02:06PM +0800, Chao Leng wrote:
>>>> Now the default packet lifetime(CMA_IBOE_PACKET_LIFETIME) is 18.
>>>> That means the minimum ack timeout is 2 seconds(2^(18+1)*4us=2.097seconds).
>>>> The packet lifetime means the maximum transmission time of packets
>>>> on the network, the maximum transmission time of packets is closely
>>>> related to the network. 2 seconds is too long for simple lossless networks.
>>>> The packet lifetime should allow the user to adjust according to the
>>>> network situation.
>>>> So add a parameter for the packet lifetime.
>>>>
>>>> Signed-off-by: Chao Leng <lengchao@huawei.com>
>>>> ---
>>>>    drivers/infiniband/core/cma.c | 6 +++++-
>>>>    1 file changed, 5 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/infiniband/core/cma.c b/drivers/infiniband/core/cma.c
>>>> index cc2222b85c88..8e2ff5d610e3 100644
>>>> --- a/drivers/infiniband/core/cma.c
>>>> +++ b/drivers/infiniband/core/cma.c
>>>> @@ -50,6 +50,10 @@ MODULE_LICENSE("Dual BSD/GPL");
>>>>    #define CMA_IBOE_PACKET_LIFETIME 18
>>>>    #define CMA_PREFERRED_ROCE_GID_TYPE IB_GID_TYPE_ROCE_UDP_ENCAP
>>>> +static unsigned char cma_packet_lifetime = CMA_IBOE_PACKET_LIFETIME;
>>>> +module_param_named(packet_lifetime, cma_packet_lifetime, byte, 0644);
>>>> +MODULE_PARM_DESC(packet_lifetime, "max transmission time of the packet");
>>>
>>> No new module parameters
>>>
>>> Maybe something in netlink would be appropriate, I'm not sure how
>>> best to deal with this.
>>>
>>> Really, the entire retransmit strategy in CM is not suitable for
>>> ethernet networks, this is just one symptom.
>> What do you think to change the CMA_IBOE_PACKET_LIFETIME to 16.
>> The maximum transmission time of packets will be about 500+ms,
>> I think this is long enough for RoCE networks.
>> 2 seconds is too long to my honest.
> 
> I don't have an informed opinion on this. I agree that 2s is too long though
> 
> Do we have any information to back up what this should be?
Assume the network is a clos topology with three layers, every packet
will pass through five hops of switches. Assume the buffer of every
switch is 128MB and the port transmission rate is 25 Gbit/s,
the maximum transmission time of the packet is 200ms(128MB*5/25Gbit/s).
Add double redundancy, it is less than 500ms.
So change the CMA_IBOE_PACKET_LIFETIME to 16,
the maximum transmission time of the packet will be about 500+ms,
it is long enough.