BerkeleyDB shared cache

Question

Hello,
I'm using BerkelyDB 4.7.25 (via the BerkeleyDB Perl module) on a FreeBSD system in a forking daemon, with a shared cache environment. Could someone, looking at the output below, tell me whether these statistics are good for the cache? You'd think that "Requested pages found in the cache (99%)" looks good, but I see no shared memory segments being used of any kind; or how much shared memory is being used, or if, even.
Thanks,
- Mark
{root} % /usr/local/bin/db_stat -m -h /var/db/smtpd/
20MB 1KB 752B Total cache size
1 Number of caches
1 Maximum number of caches
20MB 8KB Pool individual cache size
0 Maximum memory-mapped file size
0 Maximum open file descriptors
0 Maximum sequential buffer writes
0 Sleep after writing maximum sequential buffers
0 Requested pages mapped into the process' address space
231751 Requested pages found in the cache (99%)
410 Requested pages not found in the cache
1 Pages created in the cache
410 Pages read into the cache
3430 Pages written from the cache to the backing file
0 Clean pages forced from the cache
0 Dirty pages forced from the cache
0 Dirty pages written by trickle-sync thread
411 Current total page count
411 Current clean page count
0 Current dirty page count
2053 Number of hash buckets used for page location
232572 Total number of times hash chains searched for a page
2 The longest hash chain searched for a page
248006 Total number of hash chain entries checked for page
0 The number of hash bucket locks that required waiting (0%)
0 The maximum number of times any hash bucket lock was waited for (0%)
0 The number of region locks that required waiting (0%)
0 The number of buffers frozen
0 The number of buffers thawed
0 The number of frozen buffers freed
483 The number of page allocations
0 The number of hash buckets examined during allocations
0 The maximum number of hash buckets examined for an allocation
0 The number of pages examined during allocations
0 The max number of pages examined for an allocation
0 Threads waited on page I/O

Answer 1

Hi Mark,
Even if I'm not sure I can help you with everything (especially with Perl configuration) I didn't want for your post to remain unanswered.
liarafan wrote:
Could someone, looking at the output below, tell me whether these statistics are good for the cache?Your statistics are looking like the ones every Berkeley DB user dreams on. I'm not sure how your application scenario looks like (maybe you can tell us more) and what amount of data you use for the test (numebr of records, key/data size), compared to the size of cache, but the statistics are looking great. Is the cache size corresponding to the size of a normal working data set?
liarafan wrote:
I see no shared memory segments being used of any kind; or how much shared memory is being used, or if, even.Each of the BDB subsystems within an database environment is described by one or more regions ( http://www.oracle.com/technology/documentation/berkeley-db/db/ref/env/region.html ). The regions contain all of the per-process and per-thread shared information, including mutexes, that comprise a Berkeley DB environment. For example, one of the shared memory segments will be the transactional information for the system, and one (or more) of them will be the cache. What Berkeley DB product/flags are you using?
Please let me know if this helped and if you have other questions.
Thanks,
Bogdan