#10294: Slowness of pexpect interfaces on some machines
--------------------------+-------------------------------------------------
Reporter: SimonKing | Owner: was
Type: defect | Status: new
Priority: major | Milestone: sage-5.0
Component: interfaces | Keywords: pexpect select performance Ubuntu
Author: | Upstream: Reported upstream. Little or no
feedback.
Reviewer: | Merged:
Work_issues: |
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Originally reported at [http://groups.google.com/group/sage-
devel/browse_thread/thread/d4b2ca2fe7ee0678 sage-devel]:
Some machines suffer from a massive overhead in the following test.
{{{
def test(n):
st = singular.cputime()
ct = cputime()
wt = walltime()
for i in range(n):
a = singular(i)
print "Wall time:", walltime(wt)
print "Total CPU:", cputime(ct)+singular.cputime(st)
}}}
On bsd.math, one gets something like
{{{
sage: test(1000)
Wall time: 0.261538028717
Total CPU: 0.331918
}}}
On some other machines, one may get up to
{{{
sage: test(1000)
Wall time: 59.9999949932
Total CPU: 0.05
}}}
The reports indicate that the overhead is bad on Debian and worst on
Ubuntu; it seems to be independent of the CPU. So far, there is no other
variety of Linux or Unix known that shows such a massive overhead.
Studying the interface code reveals that the overhead is caused by at
least two calls to select.select, that are done when pexpect is waiting
for the prompt:
1. In {{{singular._synchronize()}}}
2. If garbage collection occurs, it is waited for a prompt once for each
variable that is to be deleted.
3. When the actual code is sent to singular.
This is why the overhead of the Gap interface is only one third of the
above: It does not use synchronization, and if a variable is to be deleted
then simply it may be overwritten when creating the next Gap element.
There must be ways to improve the Singular interface, so that one or two
calls to select() can be avoided.
David Kirkby suggests to try and upgrade pexpect - Sage uses version 2.0,
but the current pexpect is 2.3.
__I suggest that the Singular-specific problems and the pexpect upgrade
are dealt with on different tickets, while this ticket focuses on the
general problem of slow select() calls on some systems.__
The disadvantage of my suggestion is: What could we do to overcome a
system-dependent performance problem? Would there be a way to work around
select()? '''Would it help to leave pexpect and use expect instead?'''
I reported the problem to the pexpect developer, but there was no answer
yet. Who knows, perhaps this ticket will eventually be a "wontfix", but we
should at least try...
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/10294>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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