Measuring ElectricAccelerator Cache Efficiency

Somebody asked me the other day, “How much does the ElectricAccelerator filesystem cache reduce I/O load on my build host?” This is an interesting question, because in some cases, the impact of Accelerator caching is a big part of the performance benefit. Consider the case of ClearCase dynamic views, which have notoriously bad performance, particularly for stat() operations. By reducing the number of times the build accesses the host filesystem, Accelerator can provide a substantial performance boost. In one extreme case, I saw a build that ran 50 times faster just by using a single Accelerator agent, because the the host filesystem was so slow. In this post, I’ll show how you how to determine how much Accelerator caching is doing for your build.

To evaluate Accelerator cache efficiency, we need to compare the total amount of file accesses performed by the build with the amount that actually ends up hitting the host filesystem. The difference between these two values tells us how much of the total is served from the cache; in turn, the ratio between this difference and the total gives us the cache hit rate. For this comparison, we’ll look at the following specific types of filesystem access:

  • The number of readdir() operations.
  • The number of stat() operations.
  • The amount of data read from disk.
  • The amount of data written to disk.

Counting total file accesses

We can get the total number of accesses from agent performance metrics. We’ve looked at these once or twice before. Follow the directions here to obtain and aggregate metrics from the agents (make sure to get the latest version of the agentsummary script). Once you have that summary data, the Directory scans “to EFS” metric in the Caching section gives you the total number of readdir() operations performed by the build:

Next, you need the raw count of Lookup records from the Usage records section; this is the total number of stat() operations performed by the build:

Finally, you need the Total MB values for EFS disk reads and EFS disk writes from the Bandwidth section; these give us the total amount of data read and written by the build:

Counting host filesystem accesses

Now we need to find out how much of the I/O activity actually hit the host filesystem — all the accesses that were not serviced by the cache. You’ll find this data in the emake performance metrics, which we’ve looked at a bit previously. To obtain these metrics, you need to enable emake performance logging by adding --emake-debug=g --emake-logfile=emake.dlog to your emake command-line options. When your build completes, you’ll find the metrics in the file emake.dlog. First, we need the DirCache readdirs and DirCache stats values in the Counter values section; these give us the number of readdir() and stat() operations that hit the host filesystem, respectively:

Next, we need the To disk and From disk data from the Bandwidth section; these tell us the amount of data written to and read from the host filesystem:

Computing cache efficiency

Now we can put all the numbers together and compute the cache efficiency:

Metric Total accesses Host FS accesses Cache hit rate
readdir() operations 40 10 75%
stat() operations 125,881 890 99%
MB read from disk 295 29 90%
MB written to disk 219 168 23%

The effect is dramatic, even on the small build I used for this example. This is why some people call emake a “ClearCase accelerator”. Of course, every build will have a slightly different profile, but in general you ought to see similar results.

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Eric Melski

Eric Melski was part of the team that founded Electric Cloud and is now Chief Architect. Before Electric Cloud, he was a Software Engineer at Scriptics, Inc. and Interwoven. He holds a BS in Computer Science from the University of Wisconsin. Eric also writes about software development at http://blog.melski.net/.
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One response to “Measuring ElectricAccelerator Cache Efficiency”

  1. Outstanding info!! I will come back=)

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