Re: [dpdk-dev] [PATCH] config: default to shared library

[Neil Horman <nhorman@tuxdriver.com>]

On Wed, Mar 04, 2015 at 01:05:07PM +0200, Panu Matilainen wrote:
> On 03/04/2015 11:24 AM, Thomas Monjalon wrote:
> >Hi Panu,
> >
> >2015-03-04 08:17, Panu Matilainen:
> >>With symbol versioning its vital that developers test their code in
> >>shared library mode, otherwise we'll be playing "add the forgotten
> >>symbol export" from here to eternity.
> >
> >Yes we must improve the sanity checks.
> >A lot of options must be tested (or removed) and not only shared libs.
> >But the error you reported before (missing export of rte_eth_dev_release_port)
> >cannot be seen even with this patch.
> 
> I know, I didn't say it would have directly caught it. It would've likely
> been found earlier though, if nothing else then in testing of the new
> librte_pmd_null which clearly nobody had tried in shared lib configuration.
> 
This is accurate.  The default config is a tool, in the sense that it leverages
the implicit testing of any users who are experimenting with the DPDK.  Any
users out there using the DPDK test/example applications would have realized
something was amiss when the testpmd app refused to run with the null or pcap
pmd, since there was a missing symbol.  That "social fuzzing" has value, but it
only works if the defaults are carefully selected.  Currently, building for
shared libraries exposes more existing bugs than static libraries, and so we
should set that as our default so as to catch them.

> >It means we need more tools.
> >Though, default configuration is not a tool.
> 
> Yes, default config is not a tool, its a recommendation of sorts both for
> developers and users. It also tends to be the setup that is rarely broken
> because it happens to get the most testing :)
> 
And it is a tool (see above).

> >
> >>By defaulting to shared we should catch more of these cases early,
> >>but without taking away anybodys ability to build static.
> >
> >Shared libraries are convenient for distributions but have a performance
> >impact. I think that static build must remain the default choice.
> 

If utmost performance is the concern, isn't it reasonable to assume that users
in that demographic will customize their configuration to achieve that?  No one
assumes that something is tuned to be perfect for their needs out of the box if
their needs are extreemely biased to a single quality.  The best course of
action here is to set the default to be adventageous toward catching bugs, and
document the changes needed to bias for performance.

> For distros, this is not a matter of *convenience*, its the only technically
> feasible choice.
> 
> I didn't want to make the commit message into a shared library sermon, but
> if you look at the OSS landscape overall the common wisdom is that shared
> library benefits outweigh any performance impact by so much that static libs
> are almost nowhere to be found. I can change the text into a full-blown
> rationale why shared libraries should be the default if that makes any
> difference.
> 
Embedded applications actually do make extensive use of static linking to try
achieve greater performance, but they tend to be proprietary, and as such are
the exception that proves the rule.  Once an application itself becomes open
source, it biases toward shared libraries, because the minor performance impact
is well worth the increased manageability and security found in DSO's

Acked-by: Neil Horman <nhorman@tuxdriver.com>

> 	- Panu -
> 
>