New paper posted -- "These are not the k-mers you are looking for"

We've just posted a new paper to arXiv: "These are not the k-mers you are looking for: efficient online k-mer counting using a probabilistic data structure." We'll be submitting it to PLoS One after we wait a few days for comments from the Twittersphere and/or on Haldane's Sieve.

The paper discusses our CountMin Sketch implementation of a memory-efficient k-mer counting system, which we've been using for a long time now (first post: July 2010!). The software is called khmer, and between when we started working on the khmer software and this submission, k-mer counting has exploded in popularity, with Jellyfish, BF-Counter, DSK, KMC, and Turtle all joining the venerable Tallymer software in this space. As you might expect, we got a little bit worried that we were going to be late to the party. ...this paper is us bringing the dessert, or at least refreshing the punchbowl :).

I think the paper is interesting for a few different reasons.

First, the approach, and the software we built around it (khmer, are at the core of our research: partitioning and digital normalization were built on top of this approach. This paper is a long-awaited addition to those.

Second, it turns out that khmer is one of the few k-mer counting libraries to support online k-mer counting: many of the other libraries are built to read through all the data once, and then answer questions about the data. (BF-Counter is the only published k-mer counter that does this at scale, but the current version is quite slow.) This was very important for us because streaming approaches like digital normalization depend on having immediate updated access to k-mer counts; for example, single-pass diginorm requires online k-mer counting (which might help explain why Trinity, which uses Jellyfish for their normalization approach, uses an offline algorithm instead).

To put it another way, khmer gives circumstantial evidence that API is destiny. I don't think we would have developed diginorm if we'd developed an offline API. As a programmer, this is a deeply pleasing observation: good (flexible) APIs lead to more flexible research. Or, to put it yet another way, "Better Science through Superior Software!" While this is mostly applied to people focused on sustainable software, it also applies to situations where good API design can open up new research directions.

Third, we show that probabilistic approaches are the bomb for k-mer analysis. Approaches like the CountMin Sketch and the Bloom filter (as used in BF-Counter) rely on left-skewed probability distributions for efficiency. As this paper shows in detail, that perfectly describes k-mer abundance distributions in shotgun Illumina datasets; hence the use of a CountMin Sketch is not only appropriate but actually works extremely well.

Fourth, we show that khmer scales about as well as other "good" k-mer counters like Jellyfish and DSK. I'm not sure we did a fantastic job of discussing the tradeoffs but it's clear that we now have an excellent range of good choices for k-mer counters, and khmer is one of them.

Fifth, as with several of our other papers -- most notably the diginorm paper -- this is a completely reproducible paper. (In fact, I think this one is better than the diginorm paper, because more of it is automated. See our tutorial for running this on Amazon.)

And let me just say... it's not that hard to make papers reproducible, even with gobs of data. It took a certain amount of work for Qingpeng to figure it all out, but that was because I hadn't been pushing him enough from the beginning. If we'd started earlier the whole reproducibility & automated pipeline approach would have been no sweat.

Sixth, we now have a nice standard benchmarking pipeline for k-mer counters. So, coming up soon: understanding computational bottlenecks for various things, and, of course, see my older blog post on this. Moreover, we (and others!) can now quickly spot-check and cross-compare the performance of new k-mer counters using this pipeline.

One minor regret is that it's not that easy to add something to the pipeline. We focused far more on getting the paper out than on making the pipeline easy to modify.

Seventh, and the sleeper surprise -- diginorm is really efficient at high false positive rates! Richard Smith bugged me about a snarky comment we had in the diginorm source code (see github issue) where we warned people not to change the max threshold for diginorm false positive rate. This was because, until recently, we had no idea what a good FP rate was! It turns out that for diginorm you can get by with an 80% false positive rate without suffering much, at least in terms of the E. coli genome assembly; this is because the median k-mer abundance used by diginorm is very refractory to high FP rates.

Eighth, I think the title is really awesome. Dr. Tracy Teal gets credit for that one, even if we can't get it past reviewers. (My best idea was "Count me maybe: ...")

Enjoy!

--titus

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