On further inspection, I think this is unsafe: if the source files are
overwritten after the foreground task exits, but before the background
task hashes the file, then the result can be hashed incorrectly.
Unfortunately, this sort of thing happens quite commonly in autoconf tests.
I think I'm going to have to withdraw the suggestion, at least until
I've looked at it more closely.
Andrew
On 20/11/12 21:51, Andrew Stubbs wrote:
Hi all,
This isn't meant to be a final patch, but more of a starting point up
for discussion. Before it's a finished patch it'll need some autoconf
work, some adjustment to the test-cases, and probably a config option to
turn it off. (Another question is whether to do it on cache miss, as in
the patch, or in all cases to take the stats update into the background?)
The idea is simple: don't spend foreground time doing work that isn't on
the critical path.
My profiling (using valgrind --tool=callgrind) suggests that ccache
spends approximately 10% of its run time running manifest_put, and this
happens *after* ccache has already acquired all the results the calling
build needs to continue.
The time isn't wasted, it's an investment in the future, but it's still
slowing down the current build, and a build with cache-misses is already
a slow build.
I've done some basic wall-clock benchmarking, and without this patch
binutils 2.22 takes 2m:39s to build with an empty cache, on my machine,
compared to 2m:25s if I use this patch. (The measurements were not
terribly scientific, and repeated runs gave fairly noisy results, but
those times are fairly representative.)
In addition to just being nice-to-have on its own, this might also open
up some previously-too-expensive possibilities for fixing the flaws in
direct-mode caching, or even making improvements.
Thoughts?
Andrew
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