Am Mittwoch, den 17.05.2017, 14:18 -0400 schrieb David Miller: > From: Bjørn Mork <bjorn@xxxxxxx> > Date: Tue, 16 May 2017 20:24:10 +0200 > > > 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 > > When there isn't memory pressure this will hurt performance of > course. > > It is a quite common paradigm to back down to 0 order memory requests > when higher order ones fail, so this isn't such a bad change from the > perspective. > > However, one negative about it is that when the system is under memory > stress it doesn't help at all to keep attemping high order allocations > when the system hasn't recovered yet. In fact, this can make it > worse. This makes me wonder why there is no notifier chain for this. Or am I just too stupid to find it? Regards Oliver -- 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
