[SPARK-19991][CORE][YARN] FileSegmentManagedBuffer performance improvement

[srowen]

What changes were proposed in this pull request?

Avoid NoSuchElementException every time ConfigProvider.get(val, default) falls back to default. This apparently causes non-trivial overhead in at least one path, and can easily be avoided.

See #17329

How was this patch tested?

Existing tests

[srowen]

CC @witgo @vanzin

[SparkQA]

Test build #75609 has finished for PR 17567 at commit 3828d03.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[vanzin]

LGTM.

[SparkQA]

Test build #3646 has finished for PR 17567 at commit 3828d03.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[witgo]

LGTM.
Are there any performance test reports?

[srowen]

I don't have a benchmark. I was going by your report that this was a hotspot. It is a simple enough change that it seemed OK to make even with minimal runtime benefit.

[witgo]

OK, I see.

[srowen]

Merged to master