On 2020-12-03, Andrew Lunn <and...@lunn.ch> wrote: >> I don't think there's any way I could justify using a kernel that >> doesn't have long-term support. > > 5.10 is LTS. Well, it will be, once it is actually released!
Convincing people to ship an unreleased kernel would be a whole 'nother bucket of worms. It's all moot now. The decision was just made to shelve the 5.4 kernel "upgrade" and stick with 2.6.33 for now. >> [It looks like we're going to have to abandon the effort to use >> 5.4. The performance is so bad compared to 2.6.33.7 that our product >> just plain won't work. We've already had remove features to the get >> 5.4 kernel down to a usable size, but we were prepared to live with >> that.] > > Ah. Small caches? Yep. It's An old Atmel ARM926T part (at91sam9g20) with 32KB I-cache and 32KB D-cache. A simple user-space multi-threaded TCP echo server benchmark showed a 30-50% (depending on number of parallel connections) drop in max throughput. Our typical applications also show a 15-25% increase in CPU usage for an equivalent workload. Another problem is high latencies with 5.4. A thread that is supposed to wake up every 1ms works reliably on 2.6.33, but is a long ways from working on 5.4. I asked on the arm kernel mailing list if this was typical/expected, but the post never made it past the moderator. > The OpenWRT guys make valid complaints that the code > hot path of the network stack is getting too big to fit in the cache > of small systems. So there is a lot of cache misses and performance is > not good. If i remember correctly, just having netfilter in the build > is bad, even if it is not used. We've already disabled absoltely everything we can and still have a working system. With the same features enabled, the 5.4 kernel was about 75% larger than a 2.6.33 kernel, so we had to trim quite a bit of meat to get it to boot on existing units. We also can't get on-die ECC support for Micron NAND flash working with 5.4. Even it did work, we'd still have to add the ability to fall-back to SW ECC on read operations for the sake of backwards compatibility on units that were initially shipped without on-die ECC support enabled. -- Grant
