String construction is a big performance issue for JDBC drivers. Most
queries return some number of Strings. The overwhelming majority of
those Strings will be short lived. The cost of constructing these
Strings from network bytes is a large fraction of total execution
time. Any increase in the cost of constructing a String will far out
weigh any reduction in memory use, at least for query results.
All of the proposed compression methods require an additional scan of
the entire string. That's exactly the wrong direction. Something like
the following pseudo-code is common inside a driver.
{
char[] c = new char[n];
for (i = 0; i < n; i++) c[i] = charSource.next();
return new String(c);
}
The array copy inside the String constructor is a significant
fraction of JDBC driver execution time. Adding an additional scan on
top of it is making things worse regardless of the transient benefit
of more compact storage. In the case of a query result the String
will be likely never be promoted out of new space; the benefit of
compression would be minimal.
I don't dispute that Strings occupy a significant fraction of the
heap or that a lot of those bytes are zero. And I certainly agree
that reducing memory footprint is valuable, but any worsening of
String construction time will likely be a problem.
Douglas
At 02:13 PM 12/2/2014, core-libs-dev-requ...@openjdk.java.net wrote:
Date: Wed, 03 Dec 2014 00:59:10 +0300
From: Aleksey Shipilev <aleksey.shipi...@oracle.com>
To: Java Core Libs <core-libs-dev@openjdk.java.net>
Cc: charlie hunt <charlie.h...@oracle.com>
Subject: More memory-efficient internal representation for Strings:
call for more data
Message-ID: <547e362e.5010...@oracle.com>
Content-Type: text/plain; charset=utf-8
Hi,
As you may already know, we are looking into more memory efficient
representation for Strings:
https://bugs.openjdk.java.net/browse/JDK-8054307
As part of preliminary performance work for this JEP, we have to
collect
the empirical data on usual characteristics of Strings and char[]-s
normal applications have, as well as figure out the early estimates
for
the improvements based on that data. What we have so far is written
up here:
http://cr.openjdk.java.net/~shade/density/string-density-report.pdf
We would appreciate if people who are interested in this JEP can
provide
the additional data on their applications. It is double-interesting
to
have the data for the applications that process String data outside
Latin1 plane. Our current data says these cases are rather rare.
Please
read the current report draft, and try to process your own heap
dumps
using the instructions in the Appendix.
Thanks,
-Aleksey.
