Implement a DiskMRU class

[Janik Zikovsky]

This class is to be used by the file-back-end of MDEventWorkspace, but build it more generally. This is a class that:

• Limits the amount of objects in the cache to a certain amount of memory (not a fixed number of items) since objects will have varied sizes.
• Keeps the most recently used objects in memory.
• Delegates the loading/saving of the data to the object itself (because the object will stay in memory but its contents won't).
  • Use an ISaveable simple interface to delegate the loading and saving.
  • Each ISaveable tells the the DiskMRU when it needs to load itself so that the MRU :
    • Marks it as recently used.
    • Frees some memory by writing out another one.

Also, the DiskMRU should:

• Combine write operations in "blocks" so that seeking is minimized.
  • A certain minimum write size will be accumulated before writing to disk.
  • Objects will be sorted by their file index position before writing.

[Janik Zikovsky]

In [13563]:

Refs #3445: Start of a DiskMRU class. But mostly this was to remove the dependency on TestHelpers of API which makes building APITest slow.

[Janik Zikovsky]

In [13564]:

Refs #3445: Fix build.

[Janik Zikovsky]

In [13565]:

Refs #3445.

[Janik Zikovsky]

In [13600]:

Refs #3445: Basic diskMRU

[Janik Zikovsky]

In [13602]:

Refs #3445: Fix build.

[Janik Zikovsky]

In [13605]:

Refs #3445: DiskMRU with a cache to save to file, with tests.

[Janik Zikovsky]

In [13608]:

Refs #3445: DiskMRU with/without a write buffer, seems thread safe.

[Janik Zikovsky]

In [13687]:

Fixes #3445: DiskMRU class should now have all the necessary functionality. Refs #3467: Implemented diskMRU into MDEventWorkspace, which now caches a certain amount of events in memory when using the file back end. First level of writable cache (can't change the # of events in a list yet).

[Wenduo Zhou]

Unit test passed.

It is better to have these two functions in unit test, too. (1) defragFreeBlocks() (2) getMemoryStr()

[Stuart Campbell]

This ticket has been transferred to github issue ​4292