netdev - Re: [PATCH net-next] net/rds: Use DMA memory pool allocation for rds

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Date:   Wed, 2 Oct 2019 13:20:22 +0800
From:   Ka-Cheong Poon <ka-cheong.poon@...cle.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev@...r.kernel.org, santosh.shilimkar@...cle.com,
        rds-devel@....oracle.com
Subject: Re: [PATCH net-next] net/rds: Use DMA memory pool allocation for
 rds_header

On 10/2/19 1:16 AM, David Miller wrote:
> From: Ka-Cheong Poon <ka-cheong.poon@...cle.com>
> Date: Mon, 30 Sep 2019 02:08:00 -0700
> 
>> Currently, RDS calls ib_dma_alloc_coherent() to allocate a large piece
>> of contiguous DMA coherent memory to store struct rds_header for
>> sending/receiving packets.  The memory allocated is then partitioned
>> into struct rds_header.  This is not necessary and can be costly at
>> times when memory is fragmented.  Instead, RDS should use the DMA
>> memory pool interface to handle this.
>>
>> Suggested-by: Håkon Bugge <haakon.bugge@...cle.com>
>> Signed-off-by: Ka-Cheong Poon <ka-cheong.poon@...cle.com>
> 
> This is trading a one-time overhead for extra levels of dereferencing
> on every single descriptor access in the fast paths.
> 
> I do not agree with this tradeoff, please implement this more
> reasonably.


The problem with the existing way of pre-allocation is
that when there are a lot of RDS connections, the call to
ib_dma_alloc_coherent() can fail because there are not
enough contiguous memory pages available.  It is causing
problems in production systems.

And the i_{recv|send|_hdrs_dma array dereferencing is done
at send/receive ring initialization and refill.  It is not
done at every access of the header.

Thanks.


-- 
K. Poon
ka-cheong.poon@...cle.com