[libvirt] [PATCH 1/7 v2] Reduce performance overhead of the global log buffer

[Eric Blake]

On 03/10/2014 09:01 AM, Daniel P. Berrange wrote:
> With the vast number of log debug statements in the code, the
> logging framework has a measurable performance impact on libvirt
> code, particularly in the daemon event loop.
> 
> The global log buffer records every single log message triggered
> whether anyone cares to see them or not. This makes it impossible
> to eliminate the overhead of printf format expansions in any of
> the logging code. It is possible to disable the global log buffer
> in libvirtd itself, but this doesn't help client side library
> code. Also even if disabled by the config file, the existence of
> the feature makes other performance improvements in the logging
> layer impossible.
> 
> Instead of logging every single message to the global buffer, only
> log messages that pass the log filters. This if libvirtd is set
> to have log_filters="1:libvirt 1:qemu" the global log buffer will
> only get filled with those messages instead of everything. This
> reduces the performance burden, as well as improving the signal
> to noise ratio of the log buffer.
> 
> As a quick benchmark, a demo program that registers 500 file
> descriptors with the event loop (eg equiv of 500 QEMU monitor
> commands), creates pending read I/O on every FD, and then runs
> virEventRunDefaultImpl() took 1 minute 40 seconds to do 51200
> iterations with nearly all the time shown against the logging
> code. After this optimization it only takes 4.6 seconds.
> 
> Signed-off-by: Daniel P. Berrange <berrange at redhat.com>
> ---
>  src/util/virlog.c | 6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)

No one else has countered the loss of the crash buffer, so I think we're
good to make this change.

ACK.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 604 bytes
Desc: OpenPGP digital signature
URL: <http://listman.redhat.com/archives/libvir-list/attachments/20140317/313312b1/attachment-0001.sig>