How git affects kernel.org performance

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Some more data on how git affects kernel.org...

During extremely high load, it appears that what slows kernel.org down more than anything else is the time that each individual getdents() call takes. When I've looked this I've observed times from 200 ms to almost 2 seconds! Since an unpacked *OR* unpruned git tree adds 256 directories to a cleanly packed tree, you can do the math yourself.

I have tried reducing vm.vfs_cache_pressure down to 1 on the kernel.org machines in order to improve the situation, but even at that point it appears the kernel doesn't readily hold the entire directory hierarchy in memory, even though there is space to do so. I have suggested that we might want to add a sysctl to change the denominator from the default 100.

The one thing that we need done locally is to have a smart uploader, instead of relying on rsync. That, unfortunately, is a fairly sizable project.

	-hpa
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