Bug#1111052: Please increase default UDP buffer size limits
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 <josh@joshtriplett.org>
> 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.
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