[httperf] Benchmarking advice and thoughts

Mark Nottingham mnot at yahoo-inc.com
Sun May 27 19:59:21 PDT 2007


Moving to an event loop would definitely be worthwhile.

I'm not so sure about SCTP; while it's certainly interesting in an  
HTTP context, AFAIK it isn't really used for HTTP traffic yet  
(notwithstanding the experimental builds of Firefox and Apache).

WRT his comment about "friendliness" of waiting for a request to  
finish, I was under the impression that the whole idea behind httperf  
is not to do that; what's the story? It would be very useful to have  
a point-by-point comparison of current load generation tools, in  
terms of their strengths and weaknesses.

WRT testing the client vs. the server -- yes, but that's true of any  
testing tool. The benefit of httperf is that it gives enough  
information to know when this is happening, and work around it (in  
conjunction with autobench).

Cheers,


On 2007/05/28, at 7:26 AM, Ted Bullock wrote:

> Here is a cross post from the openbsd mailing list.
>
> Interesting advice.
>
> After further investigating that really spiffy libevent library I am
> working on porting httperf to libevent.
> http://en.wikipedia.org/wiki/Libevent
> http://www.monkey.org/~provos/libevent/
>
> This moves most of the portability issues away from httperf, onto the
> libevent library where they more appropriately belong.
>
> Platforms that libevent has been ported to are: Linux, *BSD, Solaris,
> Mac OS X and unofficially HP-UX
>
> Along with the improvements to portability, the use of this library
> provides a very clean mechanism to work around the FD limit.
>
> Also, his comment on multi-homing is intruiging.  SCTP is the only  
> major
> protocol that I know of that supports native mult-homing. If only just
> for the sake of keeping up with the times, I think that supporting  
> SCTP
> in httperf is useful/important.... etc.
>
> http://tinyurl.com/226lmd (SCTP paper from www2006 - XHTML)
> or
> http://www2006.org/programme/files/pdf/2035.pdf (Paper - pdf)
> http://www2006.org/programme/files/pdf/2035-slides.pdf (Slides - pdf)
> http://en.wikipedia.org/wiki/Stream_Control_Transmission_Protocol
>
> And a podcast on the topic
> http://bsdtalk.blogspot.com/2007/03/bsdtalk102-cisco-distinguished- 
> engineer.html
>
> Thoughts?
>
> -Ted
>
> Artur Grabowski wrote:
>> Ted Bullock <tbullock at canada.com> writes:
>>
>>> Theo de Raadt wrote:
>>>> One very important part of the hackathon sub-project will be to
>>>> improve 10Gb support.  Some of us believe that measuring the
>>>> performance of 10Gb networking later will help us spot some
>>>> performance problems that can improve 1Gb ethernet speed.
>>> As a side note, we recently released a new version of httperf  
>>> that now
>>> builds cleanly on OpenBSD.  It can be used to measure web system
>>> performance (including throughput).
>>>
>>> My documentation on the tool is available here
>>> http://www.comlore.com/httperf
>>>
>>> The official website is available here
>>> http://www.hpl.hp.com/research/linux/httperf/
>>>
>>> Hope it can be of some sort of use in identifying performance  
>>> bottlenecks.
>>
>> I'm sorry to say this, but last time we used httperf to optimize the
>> image servers on our site (third most page views in Sweden, see [1]),
>> we found that httperf measures more the performance of the clients
>> running the benchmark rather than the performance of the servers.
>>
>> We wrote our own tool that does what we think is the right thing. The
>> idea was to make things maximally bad for the server to see how it
>> scales under load, rather than just being nice and sending one  
>> request
>> at a time and see how it performs under optimal conditions which is
>> what httperf and most other do. When I'm back at the office (working
>> in another country right now), I'll try to get a permission from my
>> boss to release it under a free license. Until then I have a few tips
>> for you. I don't know how much httperf has evolved since then, so  
>> keep
>> that in mind, maybe some of the things are fixed and some of the  
>> issues
>> were with other benchmark tools we tested, so not everything needs to
>> apply to httperf.
>>
>> 1. Don't do any memory allocations or "rendering" of the requests
>>    while you're doing the requests. Just allocate a huge blob of
>>    memory and make sure that all requests are ready to be pushed on
>>    the wire before you start connecting to the servers. Otherwise
>>    you'll measure the performance of malloc(), the VM system, the
>>    page zeroing algorithm in the kernel and printf.
>>
>> 2. Use asynchronous connect(). You do that by setting O_NONBLOCK on
>>    the socket before connect(), then connect will return EINPROGRESS,
>>    then after you receive a write event from select() on the socket
>>    you can finish opening it. Otherwise you'll just do one connection
>>    at a time and that's not really realistic. Starting 1000  
>> connects at
>>    the same time instead of waiting one round-trip for each  
>> connection
>>    does a _lot_ to the performance you get, both positively and
>>    negatively (depending on how hard you hit the server and how well
>>    it scales). Mixing connects with writes and reads does even  
>> more evil
>>    to the servers.
>>
>> 3. Don't use select(). Select is slow and scales really badly. I
>>    suggest you simply use libevent since it provides a good
>>    abstraction for the various other good interfaces that do the same
>>    thing. On *BSD that would be kqueue(2), on linux that would be
>>    epoll(2).
>>
>> 4. Don't use gettimeofday(). clock_gettime(CLOCK_MONOTONIC, ..) is
>>    both more accurate, doesn't suffer from problems of changing time
>>    and has higher resolution.
>>
>> 5. Try to emulate slow links (this is something we didn't do in the
>>    tool, but rather in our test setup, since it was more realistic to
>>    have the ACKs delayed for real and real packet loss). The most
>>    horrible load on the servers we get is not during the normal peak
>>    hours on the days of traffic records, but rather during slow days
>>    during the summer when everyone has gone on vacations, sits in
>>    their summer house (vacations in Sweden are usually quite long and
>>    a lot people go to the country) connected with some crappy modem
>>    they have found in the attic and try to surf like usual, but on a
>>    link that's 100 times slower than what they have at home or at the
>>    office. This means lots of packet loss because of long phone lines
>>    going out to their summer house, lots of connection where the  
>> phone
>>    line drops, lots of pictures that they haven't bothered to finish
>>    downloading because it went slow and generally very long-lived
>>    connections. First time we studied this effect when we hit our
>>    capacity roof, we actually thought it was a DDoS.
>>
>> 6. Add support for multi-homed clients. Some version of Linux had a
>>    really horrible hashing algorithm for the firewall (of course we
>>    had the firewall in the test setup) and all connections from one
>>    client ended up in the same bucket. Even worse, some firewalls
>>    do load balancing (of course we had load balancing in the test
>>    setup) based on the source address without looking at the port
>>    and all connections go for the same server.
>>
>> 7. Do a DNS lookup for every connection (of course before you start
>>    making the connections) so that you get whatever DNS load  
>> balancing
>>    there is.
>>
>> All http benchmarks out there get some or all of this wrong. If
>> someonee got this right, we wouldn't have had to write our own tool.
>>
>> //art
>>
>> [1] http://preview.tinyurl.com/2eqenj
>> .
>>
>
> -- 
> Theodore Bullock, <tbullock at canada.com, tedbullock at gmail.com>
> B.Sc Software Engineering
> Bike Across Canada Adventure http://www.comlore.com/bike
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--
Mark Nottingham       mnot at yahoo-inc.com




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