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Bug
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Resolution: Fixed
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P5
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8
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linux
In the new build, on Linux, configure tries to pick up and use ccache if available on the machine. It is even being picky about the version (3.1.4 minimum) to make sure that precompiled headers are supported, since those are used in the hotspot build. I have looked into this precompiled header support in ccache some and have some insights to share. The trigger for this was observing that building hotspot with an empty ccache took a very long time compared to not using ccache at all.
Looking at the ccache manual, for precompiled headers to actually be used, the gcc flag -fpch-preprocess needs to be added to the command line. What it does is enabling precompiled headers when running gcc in preprocess only mode, which ccache needs to do in order to create the hash for the cache. I have experimented with adding this flag to see what performance differences it would yield. As a second option, I've also tried disabling precompiled headers completely. Here is a summary of the results from running on my machine (Ubuntu 10.10 x86_64 8 cores):
Summary no ccache empty cache full cache
without -fpch-preprocess 2:42 5:28 1:07
with -fpch-preprocess 2:43 3:27 1:32
no precompiled headers 3:32 3:47 0:54
Adding the flag reduces the compilation time with an empty cache tremendously. However, it also increases the compilation time with a full cache significantly. Disabling precompiled headers completely when using ccache also reduces time with the empty cache, though not as much. It does however speed up compilation with a full cache even more than any of the other options.
My conclusion is that the current situation is bad and needs to be improved. Either of the methods tested will be an improvement. Here are the options as I see them:
A. Add -fpch-preprocess to CFLAGS permanently in hotspot.
B. Have configure add -fpch-preprocess if ccache is used.
C. Have configure disable precompiled headers if ccache is used. This also removes the current version requirement of 3.1.4 on ccache for precompiled header support which is enforced in configure, and would automatically (re)enable ccache use in jprt.
I would lean towards option C, but would be interested in other opinions.
Looking at the ccache manual, for precompiled headers to actually be used, the gcc flag -fpch-preprocess needs to be added to the command line. What it does is enabling precompiled headers when running gcc in preprocess only mode, which ccache needs to do in order to create the hash for the cache. I have experimented with adding this flag to see what performance differences it would yield. As a second option, I've also tried disabling precompiled headers completely. Here is a summary of the results from running on my machine (Ubuntu 10.10 x86_64 8 cores):
Summary no ccache empty cache full cache
without -fpch-preprocess 2:42 5:28 1:07
with -fpch-preprocess 2:43 3:27 1:32
no precompiled headers 3:32 3:47 0:54
Adding the flag reduces the compilation time with an empty cache tremendously. However, it also increases the compilation time with a full cache significantly. Disabling precompiled headers completely when using ccache also reduces time with the empty cache, though not as much. It does however speed up compilation with a full cache even more than any of the other options.
My conclusion is that the current situation is bad and needs to be improved. Either of the methods tested will be an improvement. Here are the options as I see them:
A. Add -fpch-preprocess to CFLAGS permanently in hotspot.
B. Have configure add -fpch-preprocess if ccache is used.
C. Have configure disable precompiled headers if ccache is used. This also removes the current version requirement of 3.1.4 on ccache for precompiled header support which is enforced in configure, and would automatically (re)enable ccache use in jprt.
I would lean towards option C, but would be interested in other opinions.
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- Resolved
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