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https://issues.apache.org/jira/browse/VELOCITY-223?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12489434
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Nathan Bubna commented on VELOCITY-223:
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Lei has tested and confirmed memory savings, and i see no reason to doubt that
unless someone gets other results. His explanation of why it saved memory also
makes sense. He sent it to the mailing list in response to my comments above,
so i'll repost here:
On April 2, Lei Gu said:
Hi Nathan,
a) In the original code, a new copy of string image is constructed and
returned as part of the token, which is part of a node. When we cache
templates, these nodes stay in memory forever or until the template itself
is booted from the cache. We improved this by checking against the string
image pool. If the image already exists in the pool, the reference for the
image is used instead of the newly created string. The newly created string
will be garbage collected.
b) This image pool is being called constantly and that's why we don't want
to synchronized on the get call. It is okay to have one thread overwrites
the other's string image and the overwritten images won't be lost if there
could be existing references to them.
Thanks.
-- Lei
> VMs that use a large number of directives and macros use excessive amounts of
> memory - over 4-6MB RAM per form
> --------------------------------------------------------------------------------------------------------------
>
> Key: VELOCITY-223
> URL: https://issues.apache.org/jira/browse/VELOCITY-223
> Project: Velocity
> Issue Type: Bug
> Components: Engine
> Affects Versions: 1.3.1
> Environment: Operating System: All
> Platform: All
> Reporter: Christian Nichols
> Fix For: 1.6
>
> Attachments: 223-patch.txt, AllVelocityMemoryByClass.html,
> StringImagePool.java, VelocityCharStream.java, VelocityMemory.JPG
>
>
> Our application FinanceCenter is based on Velocity as the template engine.
> We
> have a library of about 200 macros and about 400 VM files. Because the
> velocity parser copies the macro body into the VM during parsing, macros that
> are frequently used (even though identical and using local contexts) use up
> large amounts of memory. On our Linux server (running Redhat 7.2 with Sun
> JDK
> 1.4.1_04) we can easily use up over 1GB of RAM simply by opening up many
> forms
> (about 150) - the server starts out using 60MB after startup. This memory
> times out after 5 minutes and is returned which tells me that it is screen
> memory. Our problem is that the NT JVM and Linux JVM (32 bit) are currently
> limited to about 1.6 - 2.0 GB of ram for heap space. Thus, using a fair
> number
> of forms in the application leaves little space for user session data.
> We have implemented a caching mechanism for compiled templates and integrated
> it into Velocity so that cached objects are timed out of the cache but the
> server is still using large amounts of memory. We finally had to rewrite
> many
> of our macros into Java so that memory usage would be reduced (note that
> these
> macros were doing complex screen formatting not business logic). Doing this
> has reduced our memory by about 30%. This is currently our biggest issue
> with
> Velocity and is causing us to review our decision to stay with Velocity going
> forward. This is because we will likely end up with close to 1,000 forms by
> the end of next year and need to know that Velocity can deal with this. Is
> there any work underway to share compiled macro AST's? This would greatly
> reduce the amount of memory used. I have reviewed the parser code that is
> doing this but it seems that this is an embedded part of the design and not
> easily changed.
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