[ccache] ccache cache in RAM -- bypassing file cache?

[Justin Lebar]

I'm not sure that ramfs copies data into the buffer cache.  According
to [1], a ramfs mount *is* the buffer cache.

[1] http://www.kernel.org/doc/Documentation/filesystems/ramfs-rootfs-initramfs.txt

On Fri, Jun 24, 2011 at 2:40 PM, David Coppit <david at coppit.org> wrote:
> We're trying to debug a performance difference of 12% speedup on a machine
> with 12GB versus a 41% speedup on a similar machine with 48GB of RAM.
>
> We've profiled the build, and believe it to have performance degradation if
> it has less than 10GB of ram. (5% slower builds with 10GB of system RAM, 10%
> slower builds with 9GB of system RAM.)
>
> Our ccache cache is in memory, on a ramfs mounted this way: sudo mount -t
> ramfs -o size=1G,mode=0700 ramfs /mnt/ramfs
>
> Our cache size after the initial warming run is 1GB.
>
> Here's what I'm thinking is happening: The OS is reading from the ramdisk
> and then caching the files in memory like it normally would. This is
> creating file cache contention with other file I/O in the build, offsetting
> our performance gain.
>
> For the machine with plenty of spare RAM, there is no file cache contention
> in the OS, and we see much better perf. Does this sound reasonable?
>
> I don't know if there's a way to prevent the OS from caching files on the
> ramfs mount, except to modify ccache's open() calls to include O_DIRECT. If
> I go that route, do I need to just modify the calls in hash.c, manifest.c,
> util.c, and gzlib.c?
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