performance of block cache

[牛兆捷 <nz...@gmail.com>]

Hi all:

I saw some interesting results from Hortonworks blog (block cache
<http://zh.hortonworks.com/wp-content/uploads/2014/03/perfeval_blockcache_v2.pdf>
).

In this result, the ratio of memory footprint to database size is held
fixed while
the absolute values are increased.

In my mind, the performance should becomes worse for larger ratio as the
increase
of the absolute value. For example BucketCache#(tmpfs), the difference
between ratio (DB"1.5":"RAM"1.0) and ratio (DB"4.5":"RAM"1.0) becomes
larger as the increase of memory.
Actually, the result of ratio ( DB"1.5":"RAM"1.0) increase linearly, and
the result of ratio (DB"1.5":"RAM"1.0) exponentially.

However, for BucketCache#(heap) and LruBlockCache, the result is out of my
expectation.
The curves of ratio (DB"1.5":"RAM"1.0) and ratio (DB"4.5":"RAM"1.0) both
increase exponentially, but the relative differences as the increase of
memory are not consistent.
Take LruBlockCache as an example, the difference of ratio
(DB"1.5":"RAM"1.0) and ratio (DB"4.5":"RAM"1.0) becomes smaller from 20 GB
to 50 GB, but becomes larger from 50 GB to 60 GB.

How to analysis the cause of this result, any ideas?


-- 
*Regards,*
*Zhaojie*