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Re: [Bug-wget] Behaviour of spanning to accepted domains

From: ekrell
Subject: Re: [Bug-wget] Behaviour of spanning to accepted domains
Date: Wed, 03 Jun 2015 08:02:55 -0500
User-agent: Roundcube Webmail/1.1.1

Hello Tim,

Thank you so much for your recommendation of valgrind/kcachegrind. I know that there is a significant amount of variable overhead, so this is exactly what I was looking for. I've only ever used valgrind for memory-related analysis; I was intending to use it to see the impact of a large hashtable on memory.

While I believe you regarding the choice of options, from what I can tell the wget documentation differs, stating:

"The ‘-D’ option allows you to specify the domains that will be followed, thus limiting the recursion only to the hosts that belong to these domains. Obviously, this makes sense only in conjunction with ‘-H’."

With the example: " wget -rH -Dserver.com http://www.server.com/ "

Well, the way I have my hosts list set up, I end up "canceling out" the two options since only the original domain is accepted, but that is because I only want to see how long it takes to realize that, not actually perform spanning.


On 2015-06-03 02:18, Tim Ruehsen wrote:
Hi Evan,

wget -rH -D $(cat trash/text.txt) williamstallings.com

is not what you want. Leave away the -H, else host-spanning is ON and -D will
be ignored.

I bring this up for one my two questions. Can someone recommend a better
method of performance testing?

What you want to know is how many CPU cycles does wget need to perform a defined task (if you compare make sure exactly the same files are downloaded). The measurement of the real time used depends on many time-variant side-
effects and thus two runs of wget are hardly comparable.

Use valgrind --tool=callgrind wget ...
You can use kcachegrind to display/analyse which part of wget took how many
CPU cycles.

Regards, Tim

On Tuesday 02 June 2015 17:21:48 ekrell wrote:

I recently used wget in such a way that the result disagreed with my
understanding of what should have happened. This came about during a
small programming exercise I am currently working on; I am attempting to
see if a large number of domains (from '-D' option) would be processed
more quickly by using the hashtable included in hash.c. While comparing
the speed of my hashed implementation of host checking against an
unmodified version of wget, the standard wget did not seem to respect my
list of accepted domains.

For the hash table version, I did the following:
In recur.c, I init a hashtable with all of the accepted domains from
opt.domain. Ignoring (for the moment) increased memory usage, I assumed that this would surely be faster than the current method of checking the
url's host.
However, when performing the check inside host.c's accept_domain
function, I realized that I would need to parse u->host to get just the domain component. This involves some overhead that may make hashing not
worth it. Also, during this entire operation, I am assuming that if it
would provide any significant improvement, it would have most likely
been done before my decision to try it out. Nonetheless, I've enjoyed
playing around with it.

My first couple tests were against my own website, using a list over
5000 domains. Both wget and wget-modified downloaded the same files, and
at roughly the same speed. My website is so small, that I wanted
something larger, but not so large that it would take more than a fre
minutes. I know going around and mirroring random sites is perhaps not
recommended behaviour (without a delay), but it worked.

I bring this up for one my two questions. Can someone recommend a better
method of performance testing?

Having found my target website, I went ahead and ran the two wget
versions, one after the other. When mine came out to be almost twice as
fast, I knew to assume that something was amiss. Sure enough, wget has
downloaded much more content.. and spanned to many more domains.

This is the command I ran for each:

<pathToVersion>/src/wget -rH -D $(cat trash/text.txt)

Excusing the useless use of cat, text.txt contains the massive
comma-separated list of domains.
Each of those domains is a randomly generated numeric value, expect for
the final one: williamstallings.com

Previously, whenever I ran this test against a (smaller) website, both
versions of wget would only recursively download those specified by my
single "real" domain in the list. However, this time (and I did it twice
to make sure) original wget went on to download from over 20 other

I would appreciate it if someone could explain what is going on here.
Seeing as this behaviour exists with the version I obtained from
git://git.savannah.gnu.org/wget.git as well as wget from the package
manager, I am not proclaiming "found a bug!". I imagine that I just
misunderstand what should have taken place, since I expected to only
have the single directory from williamstallings.com

Evan Krell

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