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How to: Improve processing time

Author: Henrik Bengtsson
Created on: 2008-11-04
Last updates: 2010-11-22

The time it takes to process a data set can be important, especially when the data set is large or many alternative methods are explored. This section covers some parts of the package that affect the processing time.

Working towards a local file system is much faster

Since the package relies heavily on the file system for reading and writing data and results, the performance of the file system will have a great impact on the overall processing speed. We know as a fact that working with files over a local network (LAN) can substantially slow down the processing, whereas working toward a local hard drive is much faster.

User success stories:

Enable parallel processing

It is possible to process data in parallel on a local machine or on compute cluster. To enable multicore processing on the current machine, use future::plan("multicore"). For more details, see How to 'Process data in parallel'.

Allow more memory to be used

As far as possible, the algorithms of the package are designed and implemented such that they have a bounded-memory usage. This can often be achieved by redesigning the algorithm to work with data from only a few arrays at any time. However, in some case this is not possible and instead the data has to be fitted in chunks. The fitting of multi-array probe-level models (PLMs), is such an example. For methods processing data in chunks, the size of each chunk is by default set so that only 0.5-1.5MB of RAM is used at any time. When more RAM is available, one might consider increasing the amount of memory used, by for instance,

# Use 10 times more RAM than the default settings
setOption(aromaSettings, "memory/ram", 10.0)

plm <- RmaPlm(csN)
fit(plm)

If argument ram is greater than one, more memory is used than the default (ram=1.0), and vice versa for small values. For more details, see 'Settings'. Since there in each chunk is some overhead from reading and especially writing data and results, using larger chunks will speed up the processing to some extend. There is a system-specific upper limit where the payoff is no longer noticeable.

User success stories:

Fit CN probes separately for PLMs

For the GenomeWideSNP_5 and GenomeWideSNP_6 chip types, the non-polymorphic units consists of single CN probes. Since there is only one single observation for each of these loci, the only reasonable "summarization" available is to take the probe signal as the estimate(*). Although this approach has been known for a long time, we still haven't had time to implement a general and automatic feature for this. However, in the meanwhile, we provide an ad hoc method for doing this manually, e.g.

plm <- AvgCnPlm(csN)
if (length(findUnitsTodo(plm)) > 0) fitCnProbes(plm)
fit(plm)

The method fitCnProbes() is available for all ProbeLevelModel:s and will copy the probe intensity as is to the corresponding elements and chip-effect files. This will take approximately 5-10 seconds per array. This will decrease the processing time substantially - for large data sets with several hours. The following call to fit() will detect the already fitted CN units and proceed with the rest.

Footnote: (*) For log-additive models such as RmaPlm, one might want to set the estimate to NA if the signal is non-positive. This is currently not implemented in the above approach, where instead all values are copied over as is.

Avoid using ASCII CDFs

A CDF in the text/ASCII file format is substantially slower to query than the same CDF in the binary (XDA or Calvin) file format. We strongly recommend that any processing is done use binary CDFs. In fact, unless you override the default settings, the package will detect when you try to use an ASCII CDF and throw an error telling you not to do so. An ASCII CDF can easily be converted to the binary/XDA file format using convertCdf() that is available in the affxparser package.

See also FAQ.

So, what is it that takes time?

In this part we describe some features and behaviors of the package that affects the processing time. Much of what is explained here takes place internally and there are often no options for the user to change the behavior such that it affects the speed. We do indeed have specific ideas how to improve processing time, which requires some design and implementation changes, cf. Page 'Future directions' (contributions appreciated).

Faster over time via memoization

For results that are likely to be generated multiple times, either in the same analysis or over a long period of time, the package caches those results in files (and sometime in memory) by a process called 'memoization'. In other words, the package caches computational expensive results for reuse in the future. For instance, when querying the CDF for the cell indices of a subset of units, the 2nd time you request the same set of units, the result is almost instant, e.g.

system.time(cells <- getCellIndices(cdf, units=1:1000))
##  user  system elapsed  
##  0.68    0.04    0.77

system.time(cells <- getCellIndices(cdf, units=1:1000))
##  user  system elapsed  
##  0.02    0.01    0.01

You will find the latter performance even when you restart R. Many of the internal methods and function calls are optimized this way, especially queries and calculations based on annotation data. The user do not have to think about it, because it is all taken care of behind the scenes.

For more information, see 'Caching of computational expensive tasks (memoization)'.

Creation of monocell CDFs

The very first time you analyze a new chip type, you will find that certain steps takes a bit longer than others. A significant example is when you run a probe-level model (PLM) summarization on this new chip type, e.g.

plm <- RmaPlm(csN)
fit(plm)

In order to store the probe summaries, a template referred to as a 'monocell CDF' is needed. If missing, it is automatically generated from the (main) CDF. This process can takes several minutes or more. Since this is done only once, the next time you analyze this chip type, the monocell CDF will be available momentarily. The monocell CDF is saved in the same directory as the main CDF and it is possible to reuse an existing monocell CDF elsewhere by copying it from one system to another.

See also FAQ.