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https://issues.apache.org/jira/browse/WW-5147?focusedWorklogId=681067&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-681067
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ASF GitHub Bot logged work on WW-5147:
--------------------------------------
Author: ASF GitHub Bot
Created on: 13/Nov/21 06:56
Start Date: 13/Nov/21 06:56
Worklog Time Spent: 10m
Work Description: JCgH4164838Gh792C124B5 commented on pull request #504:
URL: https://github.com/apache/struts/pull/504#issuecomment-967794674
Hi @davoustp.
Thanks for producing this PR in response to the JIRA issue. 👍 The proposed
change addresses the JIRA issue that was raised, by adding any expression/tree
that can be "successfully parsed" to the expression cache.
The original logic (prior to the change) would only add the expression/tree
to the expression cache if the expression could be successfully executed (in
addition to being "successfully parsed"). Looking at the history of `OgnlUtil`
(as far back as I can see in the repository), it looks like the logic
considered an expression "valid" to put in the expression cache if-and-only-if
it could be successfully executed. It seems like that may have been an
intentional decision based on the original code comment ?
There may be a trade-off in execution performance (by allowing more
expressions to be considered "valid" and cached) and memory utilization (due to
the larger number of potential expressions now being cached).
It may be difficult to predict if the change in this PR might result in
unexpected memory overhead, for some applications, due to potentially increased
expression cache size.
Having expressions considered "valid" if-and-only-if they can be executed
successfully may have provided a limiting factor to keep the cache size more
manageable in the past, but that is just my speculation.
This change may be safer to attempt for the 2.6.x branch (as opposed to PR
#503 for 2.5.x), since it introduces a new behaviour for the expression cache.
Thoughts ?
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Issue Time Tracking
-------------------
Worklog Id: (was: 681067)
Time Spent: 2h 10m (was: 2h)
> OGNL valid expression is not cached and is parsed over again in some
> situations
> -------------------------------------------------------------------------------
>
> Key: WW-5147
> URL: https://issues.apache.org/jira/browse/WW-5147
> Project: Struts 2
> Issue Type: Bug
> Components: Core
> Affects Versions: 2.5.20
> Reporter: Pascal Davoust
> Priority: Major
> Fix For: 2.6, 2.5.28
>
> Time Spent: 2h 10m
> Remaining Estimate: 0h
>
> Profiling an enterprise application under high load shows an unlikely number
> of invocations to method {{ognl.Ognl.parseExpression}} (representing a
> significant cumulated CPU time), despite having the OGNL expression cache
> enabled.
> Knowing that there is no dynamic expression in this application, this is
> puzzling: once each expression have been encountered, its parsed/compiled
> representation should be cached, avoiding the parsing phase entirely to only
> keep the execution phase.
> After investigation, this is due to the caching logic in method
> {{com.opensymphony.xwork2.ognl.OgnlUtil.compileAndExecute(String, Map<String,
> Object>, OgnlTask<T>}} (same applies to
> {{com.opensymphony.xwork2.ognl.OgnlUtil.compileAndExecuteMethod(String,
> Map<String, Object>, OgnlTask<T>)}} ) :
>
> {code:java}
> private <T> Object compileAndExecute(String expression, Map<String, Object>
> context, OgnlTask<T> task) throws OgnlException {
> Object tree;
> if (enableExpressionCache) {
> tree = expressions.get(expression);
> if (tree == null) {
> tree = Ognl.parseExpression(expression);
> checkEnableEvalExpression(tree, context);
> }
> } else {
> tree = Ognl.parseExpression(expression);
> checkEnableEvalExpression(tree, context);
> }
> final T exec = task.execute(tree);
> // if cache is enabled and it's a valid expression, puts it in
> if (enableExpressionCache) {
> expressions.putIfAbsent(expression, tree);
> }
> return exec;
> } {code}
>
>
> As shown above, on cache miss, the expression is parsed, then executed, and
> added to the cache {+}only after execution{+}.
> Problem is that the execution can fail (especially that Ognl makes use of
> exceptions quite extensively): in this case, the parsed AST is lost and not
> added to the cache.
> As a result, the same expression will go through the cache miss code path
> next time.
>
> Unless I'm mistaken, the AST of the parsed expression could be added to the
> cache right after parsing and validation, and before execution fires: an AST
> is obviously independant from any execution context so it can be reused over
> and over again.
>
> As a proof of concept, I patched our enterprise application with the changes
> above and ran the benchmark again: we experienced a very significant
> improvement in response times and CPU usage, between 10% to 30% decreased
> response times (knowing that these pages also perform time consuming tasks
> such as database updates and search engine lookups).
> I'm happy to provide a PR for you guys to have a look if you deem it worthy.
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