On Mon, Sep 07, 2020 at 07:18:48PM +0200, Paolo Abeni wrote: >Hi, > >On Mon, 2020-09-07 at 22:44 +0800, Dust Li wrote: >> We encoutered udp packets drop under a pretty low pressure >> with net.ipv4.udp_mem[0] set to a small value (4096). >> >> After some tracing and debugging, we found that for udp >> protocol, __sk_mem_raise_allocated() will possiblly drop >> packets if: >> udp_mem[0] < udp_prot.memory_allocated < udp_mem[2] >> >> That's because __sk_mem_raise_allocated() didn't handle >> the above condition for protocols like udp who doesn't >> have sk_has_memory_pressure() >> >> We can reproduce this with the following condition >> 1. udp_mem[0] is relateive small, >> 2. net.core.rmem_default/max > udp_mem[0] * 4K > >This looks like something that could/should be addressed at >configuration level ?!? Thanks a lot for the review !
Sorry, maybe I haven't make it clear enough The real problem is the scability with the sockets number. Since the udp_mem is for all UDP sockets, with the number of udp sockets grows, soon or later, udp_prot.memory_allocated will exceed udp_mem[0], and __sk_mem_raise_allocated() will cause the packets drop here. But the total udp memory allocated may still far under udp_mem[1] or udp_mem[2] > >udp_mem[0] should accomodate confortably at least a socket. Yeah, I agree udp_mem[0] should be large enough for at least a socket. Here I use 4096 just for simple and reproduce what we met before. I changed my test program a bit: - with 16 server sockets - with 1 client sending 3000 messages(size: 4096bytes) to each of those 8 server sockets - set net.core.rmem_default/max to (2*4096*4096) - and keep udp_mem unset, which by default on my 4GB VM is 'net.ipv4.udp_mem = 91944 122592 183888' https://github.com/dust-li/kernel-test/blob/master/exceed_udp_mem_min_drop/multi_sockets_test.sh Actually, with more udp sockets, I can always make it large enough to exceed udp_mem[0], and drop packets before udp_mem[1] and udp_mem[2]. > >Cheers, > >Paolo
