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From "Christopher Currens (JIRA)" <>
Subject [jira] [Created] (LUCENENET-495) Use of DateTime.Now causes huge amount of System.Globalization.DaylightTime object allocations
Date Fri, 15 Jun 2012 18:04:42 GMT
Christopher Currens created LUCENENET-495:

             Summary: Use of DateTime.Now causes huge amount of System.Globalization.DaylightTime
object allocations
                 Key: LUCENENET-495
             Project: Lucene.Net
          Issue Type: Bug
          Components: Lucene.Net Core
    Affects Versions: Lucene.Net 2.9.4, Lucene.Net 3.0.3
            Reporter: Christopher Currens
            Assignee: Christopher Currens
            Priority: Critical
             Fix For: Lucene.Net 3.0.3

This issue mostly just affects RAMDirectory.  However, RAMFile and RAMOutputStream are used
in other (all?) directory implementations, including FSDirectory types.

In RAMOutputStream, the file last modified property for the RAMFile is updated when the stream
is flushed.  It's calculated using {{DateTime.Now.Ticks / TimeSpan.TicksPerMillisecond}}.
 I've read before that Microsoft has regretted making DateTime.Now a property instead of a
method, and after seeing what it's doing, I'm starting to understand why.  DateTime.Now is
returning local time.  In order for it to calculate that, it has to get the utf offset for
the machine, which requires the creation of a _class_, System.Globalization.DaylightTime.
 This is bad for performance.

Using code to write 10,000 small documents to an index (4kb sizes), it created 1,570,157 of
these DaylightTime classes, a total of 62MB of extra memory...clearly RAMOutputStream.Flush()
is called a lot.

A fix I'd like to propose is to change the RAMFile from storing the LastModified date to UTC
instead of local.  DateTime.UtcNow doesn't create any additional objects and is very fast.
 For this small benchmark, the performance increase is 31%.

I've set it to convert to local-time, when {{RAMDirectory.LastModified(string name)}} is called
to make sure it has the same behavior (tests fail otherwise).  Are there any other side-effects
to making this change?

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