David Laight <David.Laight@xxxxxxxxxx> 於 2019年7月26日 週五 下午5:23寫道： > > From: Jian-Hong Pan > > Sent: 26 July 2019 07:18 > ... > > > While allocating all 512 buffers in one block (just over 4MB) > > > is probably not a good idea, you may need to allocated (and dma map) > > > then in groups. > > > > Thanks for reviewing. But got questions here to double confirm the idea. > > According to original code, it allocates 512 skbs for RX ring and dma > > mapping one by one. So, the new code allocates memory buffer 512 > > times to get 512 buffer arrays. Will the 512 buffers arrays be in one > > block? Do you mean aggregate the buffers as a scatterlist and use > > dma_map_sg? > > If you malloc a buffer of size (8192+32) the allocator will either > round it up to a whole number of (often 4k) pages or to a power of > 2 of pages - so either 12k of 16k. > I think the Linux allocator does the latter. > Some of the allocators also 'steal' a bit from the front of the buffer > for 'red tape'. > > OTOH malloc the space 15 buffers and the allocator will round the > 15*(8192 + 32) up to 32*4k - and you waste under 8k across all the > buffers. > > You then dma_map the large buffer and split into the actual rx buffers. > Repeat until you've filled the entire ring. > The only complication is remembering the base address (and size) for > the dma_unmap and free. > Although there is plenty of padding to extend the buffer structure > significantly without using more memory. > Allocate in 15's and you (probably) have 512 bytes per buffer. > Allocate in 31's and you have 256 bytes. > > The problem is that larger allocates are more likely to fail > (especially if the system has been running for some time). > So you almost certainly want to be able to fall back to smaller > allocates even though they use more memory. > > I also wonder if you actually need 512 8k rx buffers to cover > interrupt latency? > I've not done any measurements for 20 years! Thanks for the explanation. I am not sure the combination of 512 8k RX buffers. Maybe Realtek folks can give us some idea. Tony Chuang any comment? Jian-Hong Pan
