linux-kernel - Re: [PATCH net-next 0/2] Change the upper boundary of SMC-R's snd_buf and rcv

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Date: Wed, 29 May 2024 18:28:33 +0200
From: Wenjia Zhang <wenjia@...ux.ibm.com>
To: Guangguan Wang <guangguan.wang@...ux.alibaba.com>, jaka@...ux.ibm.com,
        davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        pabeni@...hat.com
Cc: kgraul@...ux.ibm.com, alibuda@...ux.alibaba.com, tonylu@...ux.alibaba.com,
        guwen@...ux.alibaba.com, linux-s390@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 0/2] Change the upper boundary of SMC-R's snd_buf
 and rcv_buf to 512MB



On 28.05.24 15:51, Guangguan Wang wrote:
> SMCR_RMBE_SIZES is the upper boundary of SMC-R's snd_buf and rcv_buf.
> The maximum bytes of snd_buf and rcv_buf can be calculated by 2^SMCR_
> RMBE_SIZES * 16KB. SMCR_RMBE_SIZES = 5 means the upper boundary is 512KB.
> TCP's snd_buf and rcv_buf max size is configured by net.ipv4.tcp_w/rmem[2]
> whose defalut value is 4MB or 6MB, is much larger than SMC-R's upper
> boundary.
> 
> In some scenarios, such as Recommendation System, the communication
> pattern is mainly large size send/recv, where the size of snd_buf and
> rcv_buf greatly affects performance. Due to the upper boundary
> disadvantage, SMC-R performs poor than TCP in those scenarios. So it
> is time to enlarge the upper boundary size of SMC-R's snd_buf and rcv_buf,
> so that the SMC-R's snd_buf and rcv_buf can be configured to larger size
> for performance gain in such scenarios.
> 
> The SMC-R rcv_buf's size will be transferred to peer by the field
> rmbe_size in clc accept and confirm message. The length of the field
> rmbe_size is four bits, which means the maximum value of SMCR_RMBE_SIZES
> is 15. In case of frequently adjusting the value of SMCR_RMBE_SIZES
> in different scenarios, set the value of SMCR_RMBE_SIZES to the maximum
> value 15, which means the upper boundary of SMC-R's snd_buf and rcv_buf
> is 512MB. As the real memory usage is determined by the value of
> net.smc.w/rmem, not by the upper boundary, set the value of SMCR_RMBE_SIZES
> to the maximum value has no side affects.
> 
Hi Guangguan,

That is correct that the maximum buffer(snd_buf and rcv_buf) size of 
SMCR is much smaller than TCP's. If I remember correctly, that was 
because the 512KB was enough for the traffic and did not waist memory 
space after some experiment. Sure, that was years ago, and it could be 
very different nowadays. But I'm still curious if you have any concrete 
scenario with performance benchmark which shows the distinguish 
disadvantage of the current maximum buffer size.

Thanks,
Wenjia

> Guangguan Wang (2):
>    net/smc: set rmb's SG_MAX_SINGLE_ALLOC limitation only when
>      CONFIG_ARCH_NO_SG_CHAIN is defined
>    net/smc: change SMCR_RMBE_SIZES from 5 to 15
> 
>   net/smc/smc_core.c | 7 ++++---
>   1 file changed, 4 insertions(+), 3 deletions(-)
>