[Sheepdog] [PATCH v5 4/8] sheep: teach sheep to use object cache

Liu Yuan namei.unix at gmail.com
Mon Mar 26 10:00:54 CEST 2012

On 03/26/2012 03:37 PM, huxinwei wrote:

> I'd like to have in-memory cache on the sheep side, instead of KVM
> side. It should be helpful to reduce the IO hits on disk while a lot
> of VM read the same object (consider OS booting or virus scanning). 
> Relying on in kernel cache is a possible solution, but the behavior
> then is very hard to predict. Also, as you mentioned earlier, we
> usually don't have a lot of memory in dom0, which makes the in-memory
> cache very hard to do for vanilla kernel.
> My .2 cents.

If you want memory hit instead of disk IO, why not directly *enlarge*
Guest memory to increase the page cache size. If the Guest requests can
be satisfied in its own page cache, the Guest doesn't need to be halted,
waiting for the requests to be satisfied in the host page cache memory.
By doing this, we

1 need extra memory copy between host page cache and guest memory
2 have to afford costly vm_entry/vm_exit and several system calls.
3 travel a long way back and forth through QEMU and sheep

Double page cache simply waste cpu cycles and memory.

You see lot of requests hit the same object, it is not sheep's fault,
we'd better get the *right* fix out of virtio_blk driver and IO
scheduler in guest kernel. (I might do it if time permitting me)

Currently, sheep object size is 4M, but our kernel just can issue
request less than 512K for default operation. A simple tune is that you
can just tune the request size to be as big as sheep object, namely, 4M
from 'max_sectors_kb' in /sys/block/vda/queue/.

This would heal the situation a little bit, but unfortunately, the
virtio_blk or sata driver with the combination effort of IO scheduler
doesn't do it well enough to merge the request into 4M chunk (in my dd
test, we can get 3M request chunk). This certainly need fixes.


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