Faster implementation of couch_file:pread_iolist
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Key: COUCHDB-762
URL: https://issues.apache.org/jira/browse/COUCHDB-762
Project: CouchDB
Issue Type: Improvement
Components: Database Core
Affects Versions: 0.11
Environment: any
Reporter: Adam Kocoloski
Priority: Minor
Fix For: 1.1
couch_file's pread_iolist function is used every time we read anything from
disk. It makes 2-3 gen_server calls to the couch_file process to do its work.
This patch moves the work done by the read_raw_iolist function into the
gen_server itself and adds a pread_iolist handler. This means that one
gen_server call is sufficient in every case.
Here are some benchmarks comparing the current method with the patch that
reduces everything to one call. I write a number of 10k binaries to a file,
then read them back in a random order from 1/5/10/20 concurrent reader
processes. I report the median/90/95/99 percentile response times in
microseconds. In almost every case the patch is an improvement.
The data was fully cached for these tests; I think that in a real-world
concurrent reader scenario the performance improvement may be greater. The
patch ensures that the 2-3 pread calls reading sequential bits of data (term
length, MD5, and term) are always submitted without interruption. Previously,
two concurrent readers could race to read different terms and cause some extra
disk head movement.
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