On Fri, 29 Jan 2021 22:49:27 +0100
Lorenzo Bianconi [off-list ref] wrote:

On Jan 29, Lorenzo Bianconi wrote:

quoted

On Jan 29, Jesper Dangaard Brouer wrote:  

quoted

On Fri, 29 Jan 2021 17:02:16 +0100
Jesper Dangaard Brouer [off-list ref] wrote:
  

quoted

quoted

+	for (i = 0; i < n_skb; i++) {
+		struct sk_buff *skb = skbs[i];
+
+		memset(skb, 0, offsetof(struct sk_buff, tail));    

It is very subtle, but the memset operation on Intel CPU translates
into a "rep stos" (repeated store) operation.  This operation need to
save CPU-flags (to support being interrupted) thus it is actually
expensive (and in my experience cause side effects on pipeline
efficiency).  I have a kernel module for testing memset here[1].

In CPUMAP I have moved the clearing outside this loop. But via asking
the MM system to clear the memory via gfp_t flag __GFP_ZERO.  This
cause us to clear more memory 256 bytes, but it is aligned.  Above
offsetof(struct sk_buff, tail) is 188 bytes, which is unaligned making
the rep-stos more expensive in setup time.  It is below 3-cachelines,
which is actually interesting and an improvement since last I checked.
I actually have to re-test with time_bench_memset[1], to know that is
better now.  

After much testing (with [1]), yes please use gfp_t flag __GFP_ZERO.  

I run some comparison tests using memset and __GFP_ZERO and with VETH_XDP_BATCH
set to 8 and 16. Results are pretty close so not completely sure the delta is
just a noise:

- VETH_XDP_BATCH= 8 + __GFP_ZERO: ~3.737Mpps
- VETH_XDP_BATCH= 16 + __GFP_ZERO: ~3.79Mpps
- VETH_XDP_BATCH= 8 + memset: ~3.766Mpps
- VETH_XDP_BATCH= 16 + __GFP_ZERO: ~3.765Mpps  

Sorry last line is:
  - VETH_XDP_BATCH= 16 + memset: ~3.765Mpps

Thanks for doing these benchmarks.

From my memset benchmarks we are looking for a 1.66 ns difference(10.463-8.803),
which is VERY hard to measure accurately (anything below 2 ns is
extremely hard due to OS noise).

VETH_XDP_BATCH=8 __GFP_ZERO (3.737Mpps) -> memset (3.766Mpps)
 - __GFP_ZERO loosing 0.029Mpps and 2.06 ns slower

VETH_XDP_BATCH=16 __GFP_ZERO (3.79Mpps) -> memset (3.765Mpps)
 - __GFP_ZERO gaining 0.025Mpps and 1.75 ns faster

I would say this is noise in the measurements.  Even-though batch=16
match the expected improvement, batch=8 goes in the other direction.

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer