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List:       kde-core-devel
Subject:    Re: Memory overhead of -fexceptions
From:       Rik Hemsley <rik () kde ! org>
Date:       2001-08-01 18:40:33
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#if Waldo Bastian
> You will have to run 10 or so instances of each to see the actual effects on 
> your free memory, since these numbers are a bit crappy (e.g. you don't know 
> how much is actually shared between processes).

Ok, 10 instances of each. Using the 'application' example this time.
Information from 'free', using the '-/+ buffers/cache -> used' section:

=== No exceptions

Before: 129876
After:  143812
Growth: 13936

=== Using exceptions

Before: 129684
After:  152744
Growth: 23060

So with -no-g++-exceptions, 13.6M is needed. Without the flag, 22.5M.

That's 0.89M per process.

> VmRSS shows that the memory actually used (non-virtual memory) increases with 
> 2100Kb, unfortunately it is not clear how much of the 1460Kb shared vm is 
> responsible for this and how much of 1316Kb non-shared vm is responsible for 
> this. It would not surprise me if all of the 1316Kb would end up here. That 
> would mean that the overhead of exceptions is 1.3Mb / process due to Qt 
> alone. (kdelibs will probably double this then)

It looks like it's not the full 1.3M / process, but 0.89M is still very
significant. If I had compiled KDE with exception support, this would be
even worse.

As I said earlier, I don't think it's likely that we'll ever use C++
exceptions, but at least this information is useful in that it shows
that -fno-exceptions is a very good idea ! I have 25 processes running
which link to libqt. > 22.25M wasted memory ? No thanks :)

Rik

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