New submission from Hugh Secker-Walker <hug...@users.sourceforge.net>:
Popen seems to be *very* costly, CPU wise, on Mac OS X. I need to read and process bytes from the stdout of another process that outputs bytes in a rate-limited fashion, e.g. 80000 bytes per second. If I use a shell pipeline and a simple Python script to read the bytes and log some details then top and time both agree that the Python script is using roughly 1 second of CPU time for every 2000 seconds of elapsed time. If I change the script to use Popen to start the first process and to read from proc.stdout, then top and time both agree that the Python script uses roughly 1 second of CPU time for every 6 seconds of elapsed time. That is, using Popen uses 333 times as much CPU as using a shell pipeline. In practice, this means that using a shell pipeline has no measureable impact on system performance, and I can run 300 concurrent pipeline jobs, whereas using Popen limits me to running 6 such concurrent jobs. This is on an up-to-date Mac OS X 10.5.8 system. The behavior happens with both the stock Python 2.5 from Apple and with Python 2.6 from Macports. On Ubuntu Linux the Popen solution is perhaps 2% slower than using a shell pipeline. I'll attach three scripts: bytestream -- simple program that sends bytes to its stdout pull0 -- Python script that reads its stdin and logs a couple of stats pull1 -- Python script that uses Popen to start bytestream and pulls from proc.stdout and runs abysmally slowly. Use this with a concurrent top to see the shell pipeline behavior: $ ./bytestream 80000 4 | time ./pull0 11000 Use this with a concurrent top to see the Popen behavior: $ time ./pull1 ./bytestream 80000 4 11000 In both cases you'll see that system and user time are approximately equal, but that they're roughly 333 times higher for pull1. Is there a clue in the fact that both times go up by about the same huge factor? -Hugh ---------- components: Library (Lib) files: bytestream messages: 106862 nosy: hughsw priority: normal severity: normal status: open title: Popen uses 333 times as much CPU as a shell pipe on Mac OS X type: behavior versions: Python 2.5, Python 2.6 Added file: http://bugs.python.org/file17517/bytestream _______________________________________ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue8873> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: http://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
