freebsd server limits question

Devin Teske devin.teske at
Mon Jan 2 19:54:35 UTC 2012

> -----Original Message-----
> From: owner-freebsd-questions at [mailto:owner-freebsd-
> questions at] On Behalf Of Muhammet S. AYDIN
> Sent: Monday, January 02, 2012 11:13 AM
> To: freebsd-questions at
> Subject: freebsd server limits question
> Hello everyone.
> My first post here and I'd like to thank everyone who's involved within the
> FreeBSD project. We are using FreeBSD on our web servers and we are very
> happy with it.
> We have an online messaging application that is using mongodb. Our members
> send messages to "the voice" show's (turkish version) contestants. Our two
> mongodb instances ended up in two centos6 servers. We have failed. So hard.
> There were announcements and calls made live on tv. We had +30K/sec visitors
> to the app.
> When I looked at the mongodb errors, I had thousands of these:
> You may be wondering why
> I'm telling you about centos. Well, we are making the switch from centos to
> freebsd FreeBSD. I would like to know what are our limits? How we can set it
> so our FreeBSD servers can handle min 20K connections (mongodb's connection
> limit)?
> Our two servers have 24 core CPUs and 32 GBs of RAM. We are also very open to
> suggestions. Please help me out here so we don't fail deadly, again.

We have similar hardware (24x core CPUs but 48GB of RAM instead of 32).

NOTE: The machine has 2x igb(4) interfaces and we're negotiating at 1000baseTX
"Gigabit" full-duplex link-speed.

We had similar problems, but have had zero problems in the past 2 months with
high-load (read below).

ASIDE: We're using FreeBSD 8.1-RELEASE-p6

We found that the following tweaks had to be made in /etc/sysctl.conf :

### Network Tuning ###
# Increase TCP maximum segment lifetime
# Increase TCP time before keepalive probes again
# Increase maximum number of mbuf clusters allowed (174808 => 32768)
# Increase by 8-times the maximum socket buffer size (262144 => 2097152)
# Increase by 64-times the max pending socket conn. queue size (128 => 8192)
# Increase by ~8-times the maximum number of [open] files (8232 => 65536)
# Increase by ~4-times the max files allowed open per process (7408 => 32768)
# Disable delay of ACK to try and piggyback it onto a data packet (1 => 0)
# Increase by ~2-times the maximum outgoing TCP datagram size (32768 => 65535)
# Increase maximum space for incoming UDP datagrams (41600 => 65535)
# Increase by ~6-times the maximum outgoing UDP datagram size (9216 => 57344)
# Increase by ~8-times the default stream receive space (8192 => 65535)
# Increase by ~8-times the default stream send space (8192 => 65535)

Meanwhile, yet more tweaks go into /boot/loader.conf :

### Process/Memory Tuning ###
# Increase by 4-times the maximum data size (536870912 => 2147483648)
# Increase by 4-times the maximum stack size (67108864 => 268435456)
### Network Tuning ###
# Increase maximum outgoing Netgraph datagram size (20480 => 45000)
# Increase maximum space for incoming Netgraph datagrams (20480 => 45000)
# Increase by 128-times max num of data queue items to allocate (512 => 65536)

With the above tweaks in-place for both sysctl.conf(5) and loader.conf(5), all
our problems are gone.

Your mileage may vary, but I suspect that the above collection of tweaks will
work well for you. They should be safe for both 32-bit (both regular and PAE)
and 64 (all tested). However, if you are the cautious type, I would recommend
adding one optimizer at a time, rebooting after each tweak.

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