Optimizing kernel compilation / alignments for network performance

Felix Fietkau nbd at nbd.name
Thu May 5 09:46:52 PDT 2022

On 05.05.22 18:04, Andrew Lunn wrote:
>> you'll see that most used functions are:
>> v7_dma_inv_range
>> __irqentry_text_end
>> l2c210_inv_range
>> v7_dma_clean_range
>> bcma_host_soc_read32
>> __netif_receive_skb_core
>> arch_cpu_idle
>> l2c210_clean_range
>> fib_table_lookup
> There is a lot of cache management functions here. Might sound odd,
> but have you tried disabling SMP? These cache functions need to
> operate across all CPUs, and the communication between CPUs can slow
> them down. If there is only one CPU, these cache functions get simpler
> and faster.
> It just depends on your workload. If you have 1 CPU loaded to 100% and
> the other 3 idle, you might see an improvement. If you actually need
> more than one CPU, it will probably be worse.
> I've also found that some Ethernet drivers invalidate or flush too
> much. If you are sending a 64 byte TCP ACK, all you need to flush is
> 64 bytes, not the full 1500 MTU. If you receive a TCP ACK, and then
> recycle the buffer, all you need to invalidate is the size of the ACK,
> so long as you can guarantee nothing has touched the memory above it.
> But you need to be careful when implementing tricks like this, or you
> can get subtle corruption bugs when you get it wrong.
I just took a quick look at the driver. It allocates and maps rx buffers 
that can cover a packet size of BGMAC_RX_MAX_FRAME_SIZE = 9724.
This seems rather excessive, especially since most people are going to 
use a MTU of 1500.
My proposal would be to add support for making rx buffer size dependent 
on MTU, reallocating the ring on MTU changes.
This should significantly reduce the time spent on flushing caches.

- Felix

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