[syslog-ng] TCP orig_log_iw_size='3', why?

[Balazs Scheidler]

Hi,

The default behaviour for window sizing is to divide the window into equal
sized chunks for each of your possible connections.

if your max-connections() is set to 100 and your log-iw-size() is set to
300, then each of your potential log connection would end up having 3 slots
in their window. This can easily cause performance issues, therefore
syslog-ng increases the window size to the value specified by
min-iw-size-per-reader(), which defaults to 100.

The warning message basically means that your log-iw-size() seems low
compared to your max-connections() and syslog-ng is increasing the window
size, so that each connection has 100 slots in their window.

I'd recommend simply sizing max-connections() appropriately and then use a
window size proportionate to that. A window size of 100 per connection
should be good enough for moderate traffic.

With that said, here's a breakdown of each of the values in that log
message:
* orig_log_iw_size: the value your max-connections() and log-iw-size()
determines, in case you have max-connections(100) and log-iw-size(300),
that would yield 3 in this field (300/100)
* new_log_iw_size: the result of the clamping, e.g. since 3 is too small,
syslog-ng upped the per-connection log-iw-size() to 100 (which happens to
be the value of min_iw_size_per_reader() option)
* min_iw_size_per_reader: this is a configuration option that determines
the threshold of this warning, what size window is considered too small
* min_log_fifo_size: this is the amount of messages that can be in flight
with these settings, if all your connections (of which there's 300) fill
their own window allocation (100 each), you can have 30000 messages
in-flight.

This should be less than the log-fifo-size() of your destination (each of
them, if you are routing to multiple destinations), otherwise you could
start losing data.

The only downside of a higher number of in-flight messages is queue memory
(or disk) usage. So as long as you have enough of these, you can just
increase the log-iw-size() as recommended.

NOTE: There were a few related changes in syslog-ng after this change was
added:
* flow-controlled log paths never drop messages, even if their queues would
become full. They will over-subscribe their queues instead (e.g.
log-fifo-size() is ignored if the message is being forwarded along a flow
controlled log path), this means that log-fifo-size(), is only considered
for non-flow-controlled log paths.
* a dynamic-window() sizing feature was added, which allocates the window
to individual connections in a less strict manner.


Balazs

On Sat, Mar 1, 2025 at 8:40 AM <claudio at witel.it> wrote:

> Hello syslog-ng community,
>
> I get this warning at syslog-ng start (or restart) when running journalctl
>
> [...]
> Feb 27 02:46:57 xxxsplunksyslog1 syslog-ng[2998304]:
> [2025-02-27T02:46:57.330923] WARNING: window sizing for tcp sources were
> changed in syslog-ng 3.3, the configuration value was divided by the value
> of max-connections(). The result was too small, clamping to value of
> min_iw_size_per_reader. Ensure you have a proper log_fifo_size setting to
> avoid message loss.; orig_log_iw_size='3', new_log_iw_size='100',
> min_iw_size_per_reader='100', min_log_fifo_size='30000'
> Feb 27 02:46:57 xxxsplunksyslog1 systemd[1]: Started System Logger Daemon.
>
> I thought the orig_log_iw_size value was coming from the formula
> log_iw_size/max_connections which should be 100/300=1/3
> What is this orig_log_iw_size='3' ?
>
> Thanks.
>
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-- 
Bazsi
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