[syslog-ng] High throughput UDP logging configuration.

Evan Rempel erempel at uvic.ca
Thu May 28 15:02:26 UTC 2020

On 5/28/20 7:46 AM, László Várady (lvarady) wrote:
> Notice: This message was sent from outside the University of Victoria 
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> Hi,
> > 1. The OS UDP buffer seems to be 128MB in size and the so_rcvbuf configured ins 64M 
> in size. Is that because the syslog-ng configuration of so_rcvbuf is 
> in characters but the OS buffer is in bytes?
> This is because the kernel doubles the value set by syslog-ng (to 
> allow space for bookkeeping overhead), and this doubled value is 
> returned by getsockopt(2) and other tools.


> > 3. Increasing the log_iw_size or the log_iw_size actually seems to make things worse.
> These 2 values already seem high enough.
> Disabling flow-control is also a good idea IMO, when using UDP sources.

Flow control is completely disabled. Or more precisely unspecified. It 
occurs to me that perhaps the default is enabled?

> > All suggestions that help me understand this and help to minimize the 
> drops are welcome.
> Could you share how incoming packets are distributed across the 8 sockets?
> The default SO_REUSEPORT mechanism distributes packets based on the 
> hash of (peer IP address, port) and (local IP address, port),
> Hashing collision is also likely to happen [1], so if you encounter 
> this problem, there are other possible resolutions. The commercial 
> syslog-ng version has, for example, an
> udp-balancer() driver, that uses custom BPF programs to achieve an 
> even distribution of packets.

It is a single device, single IP, single port, so there is just one 
socket that is overwhelmed. This will probably be the same failure to 
process fast enough if we move to a TCP transport.

> [1] https://blog.cloudflare.com/how-to-receive-a-million-packets/

I will read through this blog.

Evan Rempel

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