Proposed change to Falcon memory parameters

[Christopher Powers]

Hi,

The default Falcon memory parameters are:

falcon_record_memory_max         = 250 MB
falcon_page_cache_size           =   4 MB
falcon_record_scavenge_floor     =  50% (of record memory max)
falcon_record_scavenge_threshold =  67% (of record memory max)
falcon_record_chill_threshold    =   5 MB
falcon_index_chill_threshold     =   4 MB

I propose the following changes:

falcon_record_memory_max         = 250 MB
falcon_page_cache_size           = 250 MB
falcon_record_scavenge_floor     =  90% (of record memory max)
falcon_record_scavenge_threshold =  90% (of record memory max)
falcon_record_chill_threshold    =   5 MB
falcon_index_chill_threshold     =   4 MB

Reasoning:

1. Increase the page cache to 250 MB.

A properly sized page cache can make a tremendous difference in 
performance. For example, on Xeno the testcase for Bug #36442 required 5 
hours to complete with a 4 MB page cache, but less than 5 *minutes* with 
a page cache of 500 MB.

Note that the page cache size is fixed, and that the page cache is 
pre-allocated during engine initialization. The record cache is not 
pre-allocated, and grows up to the record memory max.

2. Increase record scavenge threshold to 90%.

Load-based scavenging now ensures a better response to rapid filling of 
the record cache, so we can afford to operate closer to capacity, 
keeping more records in cache--a good thing.

3. Increase record scavenge floor to 90%.

This will reduce the cost of scavenging by avoiding unnecessarily deep 
scavenges. A higher floor will also keep more records in cache.

A final note: Backlogging is enabled by scavenging, but the backlog 
operation is only triggered by transaction chill operation. So, for now, 
no matter how desperately a backlog is needed, it will not happen unless 
a transaction exceeds the chill threshold. I am revisiting this policy.


Any thoughts? I welcome your input.


Chris