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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