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While attempting to set up an Onionoo mirror I ran into memory problems with the hourly updater.

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
	at java.util.Arrays.copyOf(Arrays.java:2367)
	at java.lang.AbstractStringBuilder.expandCapacity(AbstractStringBuilder.java:130)
	at java.lang.AbstractStringBuilder.ensureCapacityInternal(AbstractStringBuilder.java:114)
	at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:480)
	at java.lang.StringBuffer.append(StringBuffer.java:309)
	at java.lang.StringBuffer.append(StringBuffer.java:300)
	at java.util.regex.Matcher.appendReplacement(Matcher.java:841)
	at java.util.regex.Matcher.replaceAll(Matcher.java:906)
	at java.lang.String.replaceAll(String.java:2162)
	at org.torproject.onionoo.docs.DocumentStore.storeDocumentFile(DocumentStore.java:278)
	at org.torproject.onionoo.docs.DocumentStore.store(DocumentStore.java:228)
	at org.torproject.onionoo.writer.DetailsDocumentWriter.updateRelayDetailsFiles(DetailsDocumentWriter.java:184)
	at org.torproject.onionoo.writer.DetailsDocumentWriter.writeDocuments(DetailsDocumentWriter.java:72)
	at org.torproject.onionoo.writer.DocumentWriterRunner.writeDocuments(DocumentWriterRunner.java:29)
	at org.torproject.onionoo.cron.Main.main(Main.java:55)

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
	at java.util.Arrays.copyOf(Arrays.java:2367)
	at java.lang.AbstractStringBuilder.expandCapacity(AbstractStringBuilder.java:130)
	at java.lang.AbstractStringBuilder.ensureCapacityInternal(AbstractStringBuilder.java:114)
	at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:415)
	at java.lang.StringBuffer.append(StringBuffer.java:237)
	at java.io.StringWriter.write(StringWriter.java:101)
	at org.apache.commons.lang.StringEscapeUtils.escapeJavaStyleString(StringEscapeUtils.java:196)
	at org.apache.commons.lang.StringEscapeUtils.escapeJavaStyleString(StringEscapeUtils.java:164)
	at org.apache.commons.lang.StringEscapeUtils.escapeJavaScript(StringEscapeUtils.java:131)
	at org.torproject.onionoo.docs.DetailsDocument.escapeJSON(DetailsDocument.java:21)
	at org.torproject.onionoo.docs.DetailsDocument.setContact(DetailsDocument.java:267)
	at org.torproject.onionoo.writer.DetailsDocumentWriter.updateRelayDetailsFiles(DetailsDocumentWriter.java:158)
	at org.torproject.onionoo.writer.DetailsDocumentWriter.writeDocuments(DetailsDocumentWriter.java:72)
	at org.torproject.onionoo.writer.DocumentWriterRunner.writeDocuments(DocumentWriterRunner.java:29)
	at org.torproject.onionoo.cron.Main.main(Main.java:55)

Neither stack trace looks like those operations would be using crazy amounts of memory, so I set up a cronjob that runs jcmd $pid GC.class_histogram once every minute. Here's the top 10 right before the JVM exited with that second stack trace:

 num     #instances         #bytes  class name
----------------------------------------------
   1:       8533341      463113472  [C
   2:       8533040      204792960  java.lang.String
   3:       3705980      148239200  java.util.TreeMap$Entry
   4:       2995129      143766192  java.util.TreeMap
   5:        593855      114020160  org.torproject.onionoo.docs.NodeStatus
   6:        604450       48686880  [Ljava.util.HashMap$Entry;
   7:       2390562       38248992  java.util.TreeSet
   8:        931813       29818016  java.util.HashMap$Entry
   9:        604451       29013648  java.util.HashMap
  10:        611484       14675616  java.lang.Long

From this profile it seems that NodeStatus would be a good candidate to save some memory. I'm attaching a branch with some memory tweaks to it as soon as I have a ticket number.