DiskMRU: track "small" boxes memory use

[Janik Zikovsky]

Caching very small boxes back and forth to disk seems to be a big slowdown, especially in the algorithms that are building up a file-backed MDEW, e.g. MergeMDEW.

I need to track the total memory used by ALL small boxes in the disk MRU in order to determine when a box should be cached or not.

The DiskMRU needs a method like: bool shouldCache(size_t memSize)

This means the MRU will have:

• A "read" buffer of most-recently read data
• A "write" buffer of data that needs to be written out but will be done in a block
• A free-space tracker for free spots in the file
• Now a "small boxes" buffer of not-cached boxes.

[Janik Zikovsky]

In [14110]:

Refs #3587: Renamed some fields in DiskMRU

[Janik Zikovsky]

In [14130]:

Refs #3587: DiskMRU flag to NOT use the MRU

[Janik Zikovsky]

In [14138]:

Refs #3587: DiskMRU tracking small objects; LoadMDEW sets the cache size to load small boxes and keeps them in memory.

[Janik Zikovsky]

In [14140]:

Refs #3587: Fix build.

[Owen Arnold]

Unit tests look to provide good coverage and all run.

[Stuart Campbell]

This ticket has been transferred to github issue ​4434