Re: [Qemu-devel] Modularizing QEMU RFC

[Alex Bennée]

Marc Marí <address@hidden> writes:

> On Mon, 3 Aug 2015 16:22:34 +0800
> Fam Zheng <address@hidden> wrote:
>
>> On Mon, 08/03 09:52, Marc Marí wrote:
>> > So any other ideas to reduce the library overhead are appreciated.
>> 
>> It would be interesting to see your profiling on the library loading
>> overhead. For example, how much does it help to reduce the library
>> size, and how much does it help to reduce the # of libraries?
<snip>
>
> Some profiling:
>
> A QEMU with this configuration:
> ./configure --enable-sparse --enable-sdl --enable-gtk --enable-vte \
>  --enable-curses --enable-vnc --enable-vnc-{jpeg,tls,sasl,png,ws} \
>  --enable-virtfs --enable-brlapi --enable-curl --enable-fdt \
>  --enable-bluez --enable-kvm --enable-rdma --enable-uuid --enable-vde \
>  --enable-linux-aio --enable-cap-ng --enable-attr --enable-vhost-net \
>  --enable-vhost-scsi --enable-spice --enable-rbd --enable-libiscsi \
>  --enable-smartcard-nss --enable-guest-agent --enable-libusb \
>  --enable-usb-redir --enable-lzo --enable-snappy --enable-bzip2 \
>  --enable-seccomp --enable-coroutine-pool --enable-glusterfs \
>  --enable-tpm --enable-libssh2 --enable-vhdx --enable-quorum \
>  --enable-numa --enable-tcmalloc --target-list=x86_64-softmmu
>
> Has dependencies on 142 libraries. It takes 60 ms between the run and
> the jump to the main function, and 80 ms between the run and the
> first kvm_entry.
>
> A QEMU with the same configuration and --enable-modules has
> dependencies on 125 libraries. It takes 20 ms between the run and the
> jump to the main function, and 100 ms between the run and the first
> kvm_entry.
>
> The libraries that are not loaded are: libiscsi, libcurl, librbd,
> librados, ligfapi, libglusterfs, libgfrpc, libgfxdr, libssh2, libcrypt,
> libidin, libgssapi, liblber, libldap, libboost_thread, libbost_system
> and libatomic_ops.
>
> As I already explained, the current implementation of modules loads
> the modules at startup always. That's why the QEMU setup takes longer,
> even though it uses G_MODULE_BIND_LAZY. And that's why I was proposing
> hotplugging.
>
> I don't know if loading one big library is more efficent than a lot of
> small ones, but it would make sense.

What's the actual use-case here where start-up latency is so important?
If it is an ephemeral cloudy thing then you might just have a base QEMU
with VIRT drivers and one big .so call "the-rest.so"?

I don't wish to disparage the idea but certainly in emulation world the
difference of 100ms or so is neither here nor there.

-- 
Alex Bennée