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Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory low

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Jim Baxter <jim_baxter@mentor.com> writes:

> 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.
> Once the memory is compacted the CDC-NCM data can resume transmitting
> at the normal tx_max rate once again.

I must say that I don't like the additional complexity added here.  If
there are memory issues and you can reduce the buffer size to
USB_CDC_NCM_NTB_MIN_OUT_SIZE, then why don't you just set a lower tx_max
buffer size in the first place?

  echo 2048 > /sys/class/net/wwan0/cdc_ncm/tx_max




Bjørn

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[RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory low Jim Baxter <jim_baxter@mentor.com> - 2017-05-16 19:50 +0200
  Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory low Bjørn Mork <bjorn@mork.no> - 2017-05-16 20:30 +0200
    Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory  low Oliver Neukum <oneukum@suse.com> - 2017-05-17 09:50 +0200
      Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory  low "Baxter, Jim" <jim_baxter@mentor.com> - 2017-05-17 13:00 +0200
    Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel  memory low David Miller <davem@davemloft.net> - 2017-05-17 20:20 +0200
      Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory  low Oliver Neukum <oneukum@suse.com> - 2017-05-18 12:10 +0200
      Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory  low "Baxter, Jim" <jim_baxter@mentor.com> - 2017-05-22 17:50 +0200
        Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel  memory low David Miller <davem@davemloft.net> - 2017-05-22 18:00 +0200
          Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory  low Oliver Neukum <oneukum@suse.com> - 2017-05-23 10:50 +0200
            Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel  memory low David Miller <davem@davemloft.net> - 2017-05-23 17:30 +0200
              Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory  low "Baxter, Jim" <jim_baxter@mentor.com> - 2017-05-23 21:10 +0200
  RE: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory  low David Laight <David.Laight@ACULAB.COM> - 2017-05-19 13:20 +0200
    Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory low Bjørn Mork <bjorn@mork.no> - 2017-05-19 16:00 +0200
      RE: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory  low David Laight <David.Laight@ACULAB.COM> - 2017-05-19 16:50 +0200
        Re: [RFC V1 1/1] net: cdc_ncm: Reduce memory use when kernel memory  low Oliver Neukum <oneukum@suse.com> - 2017-05-22 15:30 +0200

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