core,grpclb: Resolve isAndroid only once on class loading

[slandelle]

Motivation:

When multiple NameResolvers are created, the Classloader is scanned every time trying to figure out if the Platform is Android. This expensive work could be done only once.

Modification:

Cache isAndroid resolution in a constant.

Result:

Less expensive multiple NameResolvers instantiation.

[linux-foundation-easycla]

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• ✅ login: slandelle / name: Stephane Landelle (3a1dc9e)

[slandelle]

@grpc-google-owners Any idea why the CI has been stuck for 13 hours?

[temawi]

Is there a performance issue you are trying to resolve with this optimization? This would potentially save some "handful" of calls to get the class loader and a class at channel creation time, but those are not on any time sensitive RPC call path. I'm just trying to justify the, albeit slight, increase in code complexity.

[slandelle]

@temawi our use case is non standard I reckon 😄

At Gatling, we're currently working on supporting gRPC load tests.
In the case of trying so simulate a large fleet of user-agents (eg a fleet of android apps), we need to create a large amount of ManagedChannels as we really want 1 HTTP/2 connection per virtual user so the traffic against the endpoint is realistic. In this case, isAndroid is one of the hotspots we've spotted.

[temawi]

@slandelle thanks for the background, I'll go ahead and merge this. I hope it will be helpful to you guys!

[ejona86]

LGTM

[slandelle]

Thanks guys! We've spotted several other hotspots so we'll definitely provide some other PRs in a few weeks.
Cheers