clone 1111052 -1 retitle 1111052 Please increase default UDP receive buffer size (rmem_max) tags 1111052 - moreinfo retitle -1 Please increase default UDP send buffer size (wmem_max) thanks
On Wed, Aug 20, 2025 at 08:17:44PM +0200, Ben Hutchings wrote: > On Wed, 13 Aug 2025 20:11:36 -0700 Josh Triplett <[email protected]> > wrote: > > Current uses of QUIC benefit from higher buffer size limits. Please > > consider setting: > > > > net.core.rmem_max = 7500000 > > I can see that it makes sense to increase this to allow increasing the > receive window if the QUIC implementation sets its receive window based > on this, or otherwise to reduce the risk of packets being dropped if > user-space is slow to handle them. > > I would like to see a detailed justification for the specific value, > though. I haven't seen a specific justification for that *exact* value, other than that popular userspace tools/libraries seem to request 7MiB (note: not 7MB) by default. I expect that 7340032 then got rounded up to 7500000. > > net.core.wmem_max = 7500000 > > This seems wrong. With QUIC the memory for replaying lost packets is > allocated in user-space, so I don't see any need to increase buffering > on the send side. I think that this would mostly contribute to > bufferbloat. > > > See https://github.com/quic-go/quic-go/wiki/UDP-Buffer-Sizes > > No justification is given there for increasing wmem_max. That's a fair point! I haven't seen a justification for wmem_max either, and the arguments made there only support a change to rmem_max. I've split this issue into two: one for rmem_max which has justification provided, and one for wmem_max which I've left the moreinfo tag on.
