Performance That Goes Bump in the Night

Peter Welcher
Architect, Operations Technical Advisor

I thought I’d write a quick note relating to performance problems, and since it is Halloween when I’m writing this, the title “Performance That Goes Bump in the Night” seems somehow appropriate. The triggering event is having some performance situations come up recently that all came together. They affect the Cisco 3550, 3750 and 6500 switches, and the 7301 router. 

Terry Slattery’s great blog titled Application Performance Troubleshooting contains a discussion of overruns. The background story behind that is really what made me want to write this blog. 

The basic point I have for this blog is that one needs to know the performance characteristics of the equipment you’re working. You need to know limitations stated by the vendor. And you need to do performance testing with your prospective configuration and traffic mix to see if there are any hidden gotchas to the stated performance numbers. Packet size is only the most obvious of them. 

Caveat: The following contains some moderately careful test results by various people representing their best effort at the time. I have done my best to fairly state the test conditions. I cannot guarantee accuracy. Test in your own setting if you need 100% reliable numbers that represent likely performance in your network.

Oversubscription and Cisco 4500/6500 Model Switches

The first thing you probably want to avoid is oversubscription. You need to be very aware where you put oversubscription into your network, and monitor (ah yes, those pesky network management tools nobody uses) to make sure you’re not exceeding the capacity of the port or device. The easiest way I know of to oversubscribe right now is to use 6148, 6348, or 6548 cards in a 6500, as these cards are 8:1 oversubscribed. Put your 1 Gbps servers on adjacent ports, and as soon as their combined throughput exceeds 1 Gbps, you’ll be merrily dropping packets. In a closet, not so bad unless you’ve got a bunch of stockbrokers or power users. IP videoconferencing or other video, maybe not so good. 

Conclusion #1: Avoid putting servers on 6148/6348/6548 line cards, unless you only put a couple per block of 8 port group. Better: upgrade to 6748 cards with DFC or Nexus.

In a related note, I’m not a big fan of putting 4500 / 4500-E switches in datacenters either. The 4500-E, until recently, was 2:1 oversubscribed in terms of backplane performance (24 Gbps per line card). The older 4500 model only did 6 Gbps per line card — in other words “great bottleneck for Gig-attached servers”? Now, with the Sup-7, it arguably is a cheaper closet switch with better throughput than a 6500. 

I also see a lot of 6513 switches in datacenters. Just don’t do that. Until the 6513-E, they didn’t support full throughput with the Sup720 on all slots.

Cisco 3750 Switches

Some people like 3750 model switches. You can do cool things with them, like build a stack and dual home it off different stack members. Try that with your 6500! Modular growth, a heck of a lot less costly … all Good Things! However, one of my customers (Keith you know who you are — thanks for the info!) has been testing, in regards to aggregate MPLS WAN throughput. He found a couple of mis-configured edge devices where he wasn’t getting the nominal throughput. Reportedly, Verizon had to fix a couple of mis-configured policing commands in edge devices, having not set the edge burst capacity correctly, among other things. (Do you know YOU are getting the WAN throughput you’re paying for?)

Along the way, throughput was at one point a lot lower than expected. It turned out the 3750 was only apparently capable of doing about 100 Mbps Gig port to port, as verified by an Ixia tester sending moderately sized frames. As far as can be recalled, there was minimal configuration on the 3750. The 3750-X tested out at about 300 Mbps. The total throughput went up with bigger frame sizes, as one might expect. The tester did much better on 6500 ports, so the issue was apparently not the Ixia configuration / test parameters. A Cisco partner document I have states the 3750 (original) is capable of 13 M (no units, I’m guessing packets per second). That’s about 13 Gbps total, or about 260-270 Mbps per port. If those are two-way numbers, then 130 or so one way is in the same ballpark. The tentative conclusion is that neither is stellar for 1 Gbps-attached servers. Your mileage may vary, and the usual cautions apply.

Conclusion #2: The 3750 may not be a great datacenter switch. It also may not be what you want front-ending a 500 Mbps to 1 Gbps Ethernet-based Internet connection. Test and verify for yourself — and do let me know / comment this blog if you disagree with these test results!

Cisco 7301 Routers

We (mainly a colleague I won’t name) did some 7301 testing at a customer site to verify a QoS policy. This is my recollection of what he found, based on an old-ish email thread. Along the way, it turned out a 7301 running 12.3(11)T ran at 48% CPU for traffic between two Gig ports, bi-directionally, at a packet size of 222 bytes. Double that was 100% of the Gig port utilization but 100% CPU load. Adding “trust DSCP” on input and 4 output classes dropped throughput to 150 Mbps. 

That testing also showed that a 3550 with 5 input classes, 4 of them ACL matching based, peaked at 20 Mbps on a Gig port. Removing one ACL raised that to 220 Mbps. (Sorry, no data about the number of ACL rules, probably pretty short.)

Conclusion #3: Test with the configuration you plan to use. Just because there’s a Gig port there doesn’t mean you get to use all of that bandwidth. Some features, such as QoS, may adversely impact max throughput in ways you don’t expect.

Oversubscription and Network Management

I recently worked with someone who reacted to something I said by pulling out a wallet card and telling me I’d violated “Man Rule #5”. I’m starting to wonder if a lot of male engineers think something along the lines of “Real Men don’t use network management tools”. Is that also written on one of those wallet cards?

Even using the tools, it’s hard to see oversubscription directly — sometimes overruns are a better indicator. Why you can’t see oversubscription directly: I don’t know of any tool that is going to be all that great at reporting oversubscription of ASIC-based port  groups. Is there one that will add up the inbound utilization for ports 1-8, 9-16, etc. and report it? On a Nexus 7000 32-port 10 G card, add  ports 1, 3, 5, 7 or 2, 4, 6, 8?? 

You also have to bear in mind that utilization averages lie. That is, an average is reporting a bunch of traffic spikes mixed with a lot of low traffic levels. Yet the spikes are what cause the drops — you don’t get “rollover” credits for having a lull in traffic! So Terry and I believe in 95th-percentile data, which tells you how bad things are getting while leaving out some of the most extreme behavior. And that’s a good topic for another blog.

7 responses to “Performance That Goes Bump in the Night

  1. I think your final paragraph is the most important. Way too many network engineers see their network, either when base-lining or troubleshooting though a five minute average worst case, or thirty second one, best case. There can be a lot of bursty congestion out there so watching the drops is educational, even if the averages don’t seem that high.

  2. Thanks, that’s exactly it!

    In one MAN case, we (I) saw maybe 20-25% utilization reported in some 15 minute polling. Yet when we checked, we saw output drops. I should have known better: prior experience is that the higher 5 minute averages can run double 15 minute ones, and instantaneous bursts maybe double that again.

    We also heard "but my VoIP is OK". Interesting and clever argument. Yes, but VoIP will tolerate 1 in 1000 (0.1%) drops (that was a Cisco number I saw for VoIP provider SLA’s). VoIP rides out drops pretty well. (Video is another story.) Terry’s and my prior blogging states why even 0.001% or smaller drop rates still can significantly hurt TCP throughput.

  3. One can set up a stacked graph in cacti for selected interfaces in an ASIC port group. That would allow you to then create an alert for the design threshold being exceeded.

  4. This is valuable information backed up by hard data and testing to prove the points. However, testing and testing equipment remains expensive and out of reach for nearly everybody. Regardless of vendor (Ixia, Spirent, Breaking Point are just a few), the purchase and maintenance of line rate testing equipment is beyond the budget.

    Secondly, most of this equipment is downright hard to use. Ixia & Spirent particularly seem to delight in complicated user interfaces that are full of bugs and ‘challenges’ – this drives a user experience that leads to testing avoidance.

    As a culture, testing among network engineers is not performed for these two reasons. But most importantly, is that vendors are reluctant to release information on their architectures that assist in developing test plans.

    Anyway, my 0.02c worth.

  5. Interesting, good tip. I didn’t know that Cacti could do that — never pursued the custom graphs side that much. Presumably it can be done in an inheritable way, i.e. across devices of certain types. Good stuff!

  6. I agree, testing is hard to do right. Marketing numbers versus real numbers can also be a challenge, particularly when someone isn’t into "device internals and architecture". Although sometimes I get the feeling the vendor engineers don’t necessarily know where all the bottlenecks are, e.g. strange interactions between QoS or dot1q encapsulation and routing performance. Or maybe they know but coding priorities didn’t allow them to work on that.

  7. After an email exchange triggered by the above article, I did some more research (fancy name for Google search and article skimming). I found the following useful Cisco URL: [url][/url]. I’m pasting in the relevant performance data below.

    Conclusion: There some forwarding info out there. I’m still not sure whether to divide the numbers by 2 (marketing double-counting) or not. Comparing the lab testing to the below numbers, I suspect you need to divide the below numbers by 2.

    Performance info from the above URL:

    Forwarding rate: 6.5 mpps (Cisco Catalyst 3750-24TS, 3750-24FS, and 3750-24PS), 13.1 mpps (Cisco Catalyst 3750-48TS and 3750-48PS), 17.8 mpps (Cisco Catalyst 3750G-12S), 35.7 mpps (Cisco Catalyst 3750G-24T), 38.7 mpps (Cisco Catalyst 3750G-24TS and 3750G-24WS), 35.7 mpps (Cisco Catalyst 3750G-16TD), 38.7 mpps (Cisco Catalyst 3750G-24TS-1U), 38.7 mpps (Cisco Catalyst 3750G-24PS), 38.7 mpps (Cisco Catalyst 3750G-48TS), 38.7 mpps (Cisco Catalyst 3750G-48PS)

    I usually figure 1 Mpps = about 1 Gbps of throughput, for quick and dirty comparison anyway. (It really depends in packet size, but as I recall my though process, average packet size is 128 B/packet or 1024 bits/packet, so x1000 gives approx bits/second, YMMV.) So using Cisco’s own numbers, a 3750-48 TS will do at most 13.1 Mpps or about 13 Gbps total summing traffic on all ports. And 13/48 is about 1/4 or 250 Mbps per port. If the printed numbers are doubled marketing numbers (a common practice), that’s somewhere between 125-250 Mbps per Gig port. I’m not sure if you can actually do 1 Gbps if single pair of ports in use, the lab testing my contact did were port to port so I’d guess probably NOT.

    The short version: just because it has Gig ports doesn’t mean you get to actually drive them at 1 Gbps.

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