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On Sun, Apr 27, 2014 at 6:56 PM, Joshua Kinard <kumba@g.o> wrote: |
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> |
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> My curiosity, as I have not attempted LTO yet on any machine, is what are |
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> the RAM requirements? Is it a hard limit, wherein the compiler simply fails |
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> if there isn't enough RAM, or does it just start hitting swap real hard? |
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It just allocates RAM, and the OS does the rest. I've seen it invoke |
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the OOM killer. That was back when I only had 8GB of RAM. Now I have |
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16GB and I only need to disable LTO on the really big packages. |
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Of course, if you set an appropriate ulimit then the process will just |
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terminate more gracefully. I'd highly recommend doing just that if |
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you have a lot of swap available. |
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> Those of us using older archs where the RAM is limited might have to be more |
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> cautious w/ LTO. I.e., my SGI O2 maxes right now at 512MB. It can go to |
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> 1GB if the odd memory/PROM issue is ever worked out. But 512MB is it for |
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> now, so what are my odds of successfully using LTO on that? |
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About zero. Well, I'm sure it will work fine for hello.c, especially |
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if you eliminate any function calls inside of it. |
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> |
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> Especially if LTO helps to reduce the final binary size, that's less data |
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> being shuffled around main memory and the CPU caches, which, although means |
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> slower compile times, might hake such a machine a bit snippier. Though, I |
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> dread how long GCC will take to build itself w/ LTO. The O2 already needs |
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> ~18hrs for 4.8. I haven't tried 4.9 on it yet. |
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Yeah, good luck with that... :) |
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I'd be curious as to what you find. You can always try it out by |
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picking a small package and doing a CFLAGS=foo emerge bar. Be sure to |
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only use -j1 -flto=1 as well. |
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Rich |
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