The web is an endless series of edge cases

December 17, 2009

Recently I’d been exchanging emails with Jimmy Lin at CMU. Jimmy has written up some great Hadoop info, and provided some useful classes for working with the ClueWeb09 dataset.

In one of his emails, he said:

However, what I’ve learned is that whenever you’re working with web-scale collections, it exposes bugs in otherwise seemingly solid code.  Sometimes it’s not bugs, but rather obscure corner cases that don’t happen for the most part.  Screwy data is inevitable…

I borrowed his “screwy data is inevitable” line for the talk I gave at December’s ACM data mining SIG event, and added a comment about this being the reason for having to write super-defensive code when implementing anything that touched the web.

Later that same week, I was debugging a weird problem with my Elastic MapReduce web crawling job for the Public Terabyte Datset project. At some point during one of the steps, I was getting LeaseExpiredExceptions in the logs, and the job was failing. I posted details to the Hadoop list, and got one response from Jason Venner about a similar problem he’d run into.

Is it possible that this is occurring in a task that is being killed by the framework. Sometimes there is a little lag, between the time the tracker ‘kills a task’ and the task fully dies, you could be getting into a situation like that where the task is in the process of dying but the last write is still in progress.
I see this situation happen when the task tracker machine is heavily loaded. In once case there was a 15 minute lag between the timestamp in the tracker for killing task XYZ, and the task actually going away.

It took me a while to work this out as I had to merge the tracker and task logs by time to actually see the pattern. The host machines where under very heavy io pressure, and may have been paging also. The code and configuration issues that triggered this have been resolved, so I don’t see it anymore.

This led me down the path of increasing the size of my master instance (I was incorrectly using m1.small with a 50 server cluster), increasing the number of tasktracker.http.threads from 20 to 100, etc. All good things, but nothing that fixed the problem.

However Jason’s email about merging multiple logs by timestamp value led me to go through all of the logs in more detail. And this led me to the realization that the job previous to where I was seeing a LeaseExpiredException had actually died quite suddenly. I then checked the local logs I wrote out, and I saw that this was right after a statement about parsing an “unusual” file from stanford.edu: http://library.stanford.edu/depts/ssrg/polisci/NGO_files/oledata.mso

The server returns “text/plain” for this file, when in fact it’s a Microsoft Office document. I filter out everything that’s not plain text or HTML, which lets me exclude a bunch of huge Microsoft-specific parse support jars from my Hadoop job jar. When you’re repeatedly pushing jars to S3 via a thin DSL connection, saving 20MB is significant.

But since the server lies like a rug in this case, I pass it on through to the Tika AutoDectectParser. And that in turn correctly figures out that it’s a Microsoft Office document, and makes a call to a non-existing method. Which throws a NoSuchMethodError (not an Exception!). Since it’s an Error, this flies right on by all of the exception catch blocks, and kills the job.

Looks like I need to get better at following my own advice – a bit of defensive programming would have saved me endless hours of debugging and config-thrashing.

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Performance problems with vertical/focused web crawling

May 19, 2009

Over at the Nutch mailing list, there are regular posts complaining about the performance of the new queue-based fetcher (aka Fetcher2) that became the default fetcher when Nutch 1.0 was released. For example:

Not sure if that problem is solved, I have it and reported it in a previous thread. Extremely fast fetch at the beginning and damn slow fetches after a while.

There’s also a Jira issue (NUTCH-721) filed on the problem.

But in my experience using Nutch to do vertical/focused crawls, this problem of having very slow fetch performance at the end of a crawl is a fundamental problem caused by not enough unique domains. If a crawler is polite, then once the number of unique domains drops significantly (because you’ve fetched all of the URLs for most of the domains), the fetch performance always drops rapidly, at least if your crawler is properly obeying robots.txt and the default rules for polite crawling.

Just for grins, I tracked a number of metrics at the tail end of a vertical crawl I was just doing using Bixo – that’s the vertical crawler toolkit I’ve been working on for the past two months. The system configuration (in Amazon’s EC2) is an 11 server cluster (1 master, 10 slaves) using the small EC2 instance. I run 2 reducers per server, with a maximum of 200 fetcher threads per reducer. So the theoretical maximum is 4000 active fetch threads, which is way more than I needed, but I was also testing memory usage (primarily kernel memory) of threads, so I’d cranked this way up.

I started out with 1,264,539 URLs from 41,978 unique domains, where I classify domains using the “paid level” ontology as described in the IRLbot paper. So http://www.ibm.com, blogs.us.ibm.com, and ibm.com are all the same domain.

Here’s the performance graph after one hour, which is when the crawl seemed to enter the “long tail” fetch phase…

Fetch Performance

The key things to note from this graph are:

  • The 41K unique domains were down to 1700 after an hour, and then slowly continued to drop. This directly impacts the number of simultaneous fetches that can politely execute at the same time. In fact there were only 240 parallel fetches (== 240 domains) after an hour, and 64 after three hours.
  • Conversely, the average number of URLs per domain climbs steadily, which means the future fetch rate will continue to drop.
  • And so it does, going from almost 9K/second (scaled to 10ths of second in the graph) after one hour down to 7K/second after four hours.

I think this represents a typical vertical/focused crawl, where a graph of the number of URLs/domain would show a very strong exponential decay. So once you’ve fetched the single URLs from a lot of different domains, you’re left with lots of URLs for a much smaller number of domains. And your performance will begin to stink.

The solution I’m using in Bixo is to specify the target fetch duration. From this, I can estimate the number of URLs per domain I might be able to get, and so I pre-prune the URLs put into each domain’s fetch queue. This works well for the type of data processing workflow that the current commercial users of Bixo need, where Bixo is a piece in the data processing pipeline that needs to play well (ie doesn’t stall the entire process).

Anyway, I keep thinking that perhaps some of the reported problems with Nutch’s Fetcher2 are actually a sign that the fetcher is being appropriately polite, and the comparison with the old fetcher is flawed because that version had bugs where it would act impolitely.