12.2 CPUs and Computer Architecture

The clock speed of a CPU isn't necessarily an accurate indicator of how fast it will run BLAST. There are other complicating factors such as the amount of L2 cache, the memory latency and the speed of the front-side bus. Unfortunately, there is no good rule to predict how fast BLAST will perform on a particular computer except for the obvious within-family predictions—for example, that a 1-GHz Pentium III will be faster than an 800-MHz Pentium III. The best you can do is to benchmark a bunch of systems or contact people who have already done so.

Two benchmarks are provided Table 12-3. Before reading the description, please understand that you should use extreme caution whenever interpreting any benchmarks because the benchmarking protocol may be very different from your everyday tasks, and therefore may not reflect real-world performance. The best benchmark procedure should mimic your daily routine. In addition, if you use benchmarks to decide what hardware to purchase, you may be in for a nasty surprise, as other important considerations may override a simplistic interpretation of the "most BLAST for the buck." Total cost of ownership is a complicated equation that includes maintenance, support, facilities, cooling, and interfacing with legacy equipment and culture.

Chapter 12 shows the performance on various platforms when searching all members of a database against themselves. There are two databases, and both can be found at http://examples.oreilly.com/BLAST. The tests were performed using default parameters for NCBI-BLAST. The following command lines were used:

time blastall -p blastn -d ESTs -i ESTs > /dev/null
time blastall -p blastp -d globins -i globins > /dev/null

Table 12-2. Performance benchmarks of various systems

CPU; clock speed	blastn test		blastp test
	Time (sec)	Giga-cycles	Time (sec)	Giga-cycles
Macintosh G4: 550 MHz	1011	556	1599	879
Sun Ultra Sparc III; 750 MHz	835	626	1427	1070
Intel Pentium III; 1 GHz	649	649	1187	1187
Intel Pentium IV Xeon; 1.8 GHz	469	844	788	1418
AMD Athlon 1800+; 1.533 GHz	416	638	741	1136

12.2.1 Multiprocessor Computers

One way to speed up BLAST is to employ multiprocessor computers. BLAST is a multithreaded application and can utilize the additional processors. Adding additional processors to a computer is sometimes cheaper than purchasing multiple machines because you don't have to duplicate all the other components. That said, once outside the commodity computer market, prices rise steeply, and a computer with 32 CPUs is likely to cost you much more than 16 dual-CPU computers. The improvement with multiple processors isn't completely linear, and it depends on the type of search.

If you want a single BLAST job to complete quickly, it's best to use as many CPUs as possible. On NCBI-BLAST, you can increase the number of processors with the -a option, and on WU-BLAST you use the -cpus option (see Chapter 13 and Chapter 14 for more information). However, for best aggregate performance, it is better to use only 1 CPU for each BLAST job and load up the machine with as many jobs as there are processors. If you are searching multiple databases, you may need a lot of RAM if you wish to keep them all cached.

12.2.2 Operating Systems and Compilers

Even on the same hardware, BLAST may run faster under different operating systems. Due to the complex interactions between operating systems, compilers, and computer architecture, it is difficult to predict what the optimal combination will be. If you have the time and inclination, you might be able to eke out as much as an extra 5 percent in speed. However, choosing an operating system based entirely on BLAST performance may not be a wise choice, so this is probably the last thing to consider.