[issue8873] Popen uses 333 times as much CPU as a shell pipe on Mac OS X
report at bugs.python.org
Wed Jun 2 00:18:46 CEST 2010
New submission from Hugh Secker-Walker <hughsw at 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?
components: Library (Lib)
title: Popen uses 333 times as much CPU as a shell pipe on Mac OS X
versions: Python 2.5, Python 2.6
Added file: http://bugs.python.org/file17517/bytestream
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