Jim Baxter <jim_baxter@xxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
