WT-2391: De-prioritize eviction from indexes

With commit 7f84e3b it's unlikely for a page to NOT have a read
generation, it only happens if a reader configured with WT_READ_NO_GEN
set were to read a page into cache, and that won't happen because those
readers set either WT_READ_CACHE or WT_READ_WONT_NEED.

The eviction code still has special code to handle pages without a read
generation set, and it maintains a "oldest page generation" value to
support that. However, the oldest page generation value isn't really
right -- it's simply that page generation value from the first slot in
the queue, which can be special (for example, if the page generation was
set to WT_READGEN_OLDEST because the page was empty).

Add a test in the page-read code to set the page generation if it's
never been set, regardless of WT_READ_NO_GEN.

Remove the eviction code to maintain an "oldest page generation".

Leave the eviction code to handle WT_READGEN_NOTSET (it can still see
that value if there's a race), but change it to set the page read
generation to the "standard" value instead of trying anything fancy.
That ensure that if there's a bug (we read the page but for some reason
never set the page generation), the page won't stay in that state
forever.

0 files changed, 0 insertions, 0 deletions