Re: [Qemu-devel] Question about vNVDIMM file format

[Richard W.M. Jones]

I thought you might be interested in some performance results, "hot
off the presses".

With DAX enabled, I see the following messages in the guest kernel
logs, which I assume means it is working:

[    0.469364] EXT4-fs (pmem0): DAX enabled. Warning: EXPERIMENTAL, use at your 
own risk
[    0.469932] EXT4-fs (pmem0): mounting ext2 file system using the ext4 
subsystem
[    0.470682] EXT4-fs (pmem0): mounted filesystem without journal. Opts: dax

Enabling vNVDIMM + ext4.ko + DAX for the libguestfs appliance[1] root
disk improves our boot+shutdown performance measure[2] by between 20
and 30 milliseconds, which is about 5% faster at the moment.

I also wanted to know if memory usage is reduced.  I ran `free -m'
inside a freshly booted appliance.

Without DAX:

$ free -m
              total        used        free      shared  buff/cache   available
Mem:            485           3         451           1          30         465
Swap:             0           0           0

With DAX:

$ free -m
              total        used        free      shared  buff/cache   available
Mem:            485           3         469           1          12         467
Swap:             0           0           0

I also wanted to know if qemu's memory usage is reduced.  I captured
the rusage.ru_maxrss of the qemu subprocess with and without DAX.  The
difference is only about 5 MB, which doesn't seem like very much to
me.  Perhaps I'm measuring this wrong.

If you have better suggestions for measuring memory usage, please let
me know.

Rich.

[1] http://libguestfs.org/guestfs-internals.1.html#architecture
[2] 
http://libguestfs.org/guestfs-performance.1.html#baseline:-starting-the-appliance

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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