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SubjectRe: [PATCH V2 1/1] net: cdc_ncm: Reduce memory use when kernel memory low

From: David S. Miller (
Sent: Fri, 30 Jun 2017 12:59:53 -0400
Cc:,,,,, David.Laight@ACULAB.COM
Subject: Re: [PATCH V2 1/1] net: cdc_ncm: Reduce memory use when kernel memory low

> From: Jim Baxter <>
> Date: Wed, 28 Jun 2017 21:35:29 +0100
>> The CDC-NCM driver can require large amounts of memory to create
>> skb's and this can be a problem when the memory becomes fragmented.
>> This especially affects embedded systems that have constrained
>> resources but wish to maximise the throughput of CDC-NCM with 16KiB
>> NTB's.
>> The issue is after running for a while the kernel memory can become
>> fragmented and it needs compacting.
>> If the NTB allocation is needed before the memory has been compacted
>> the atomic allocation can fail which can cause increased latency,
>> large re-transmissions or disconnections depending upon the data
>> being transmitted at the time.
>> This situation occurs for less than a second until the kernel has
>> compacted the memory but the failed devices can take a lot longer to
>> recover from the failed TX packets.
>> To ease this temporary situation I modified the CDC-NCM TX path to
>> temporarily switch into a reduced memory mode which allocates an NTB
>> that will fit into a USB_CDC_NCM_NTB_MIN_OUT_SIZE (default 2048 Bytes)
>> sized memory block and only transmit NTB's with a single network frame
>> until the memory situation is resolved.
>> Each time this issue occurs we wait for an increasing number of
>> reduced size allocations before requesting a full size one to not
>> put additional pressure on a low memory system.
>> Once the memory is compacted the CDC-NCM data can resume transmitting
>> at the normal tx_max rate once again.
>> Signed-off-by: Jim Baxter <>
> If someone could review this patch, I remember this issue being discussed
> a while ago, I would really appreciate it.


For reference this replaces the original discussion in and

Best regards,

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