On Sun, Jun 29, 2025 at 10:51 AM Jason Xing <kerneljasonxing@xxxxxxxxx> wrote: > > On Fri, Jun 27, 2025 at 7:01 PM Jason Xing <kerneljasonxing@xxxxxxxxx> wrote: > > > > From: Jason Xing <kernelxing@xxxxxxxxxxx> > > > > This patch provides a setsockopt method to let applications leverage to > > adjust how many descs to be handled at most in one send syscall. It > > mitigates the situation where the default value (32) that is too small > > leads to higher frequency of triggering send syscall. > > > > Considering the prosperity/complexity the applications have, there is no > > absolutely ideal suggestion fitting all cases. So keep 32 as its default > > value like before. > > > > The patch does the following things: > > - Add XDP_MAX_TX_BUDGET socket option. > > - Convert TX_BATCH_SIZE to tx_budget_spent. > > - Set tx_budget_spent to 32 by default in the initialization phase as a > > per-socket granular control. 32 is also the min value for > > tx_budget_spent. > > - Set the range of tx_budget_spent as [32, xs->tx->nentries]. > > > > The idea behind this comes out of real workloads in production. We use a > > user-level stack with xsk support to accelerate sending packets and > > minimize triggering syscalls. When the packets are aggregated, it's not > > hard to hit the upper bound (namely, 32). The moment user-space stack > > fetches the -EAGAIN error number passed from sendto(), it will loop to try > > again until all the expected descs from tx ring are sent out to the driver. > > Enlarging the XDP_MAX_TX_BUDGET value contributes to less frequency of > > sendto() and higher throughput/PPS. > > > > Here is what I did in production, along with some numbers as follows: > > For one application I saw lately, I suggested using 128 as max_tx_budget > > because I saw two limitations without changing any default configuration: > > 1) XDP_MAX_TX_BUDGET, 2) socket sndbuf which is 212992 decided by > > net.core.wmem_default. As to XDP_MAX_TX_BUDGET, the scenario behind > > this was I counted how many descs are transmitted to the driver at one > > time of sendto() based on [1] patch and then I calculated the > > possibility of hitting the upper bound. Finally I chose 128 as a > > suitable value because 1) it covers most of the cases, 2) a higher > > number would not bring evident results. After twisting the parameters, > > a stable improvement of around 4% for both PPS and throughput and less > > resources consumption were found to be observed by strace -c -p xxx: > > 1) %time was decreased by 7.8% > > 2) error counter was decreased from 18367 to 572 > > More interesting numbers are arriving here as I run some benchmarks > from xdp-project/bpf-examples/AF_XDP-example/ in my VM. > > Running "sudo taskset -c 2 ./xdpsock -i eth0 -q 1 -l -N -t -b 256" > > Using the default configure 32 as the max budget iteration: > sock0@eth0:1 txonly xdp-drv > pps pkts 1.01 > rx 0 0 > tx 48,574 49,152 > > Enlarging the value to 256: > sock0@eth0:1 txonly xdp-drv > pps pkts 1.00 > rx 0 0 > tx 148,277 148,736 > > Enlarging the value to 512: > sock0@eth0:1 txonly xdp-drv > pps pkts 1.00 > rx 0 0 > tx 226,306 227,072 > > The performance of pps goes up by 365% (with max budget set as 512) > which is an incredible number :) Weird thing. I purchased another VM and didn't manage to see such a huge improvement.... Good luck is that I own that good machine which is still reproducible and I'm still digging in it. So please ignore this noise for now :| Thanks, Jason
