serialize the ingestion of logfiles

performance improvement of roughly 40%

pdb-ui/src/main/java/org/lucares/pdbui/CsvUploadHandler.java

@@ -42,6 +42,12 @@ public class CsvUploadHandler implements PropertyKeys, DisposableBean {
         try {
             final ArrayBlockingQueue<Entries> queue = performanceDb.getQueue();
             for (final MultipartFile file : files) {
+
+                // insert one file at a time. This improves performance, because the likelihood
+                // of values having the same tags is greatly increased. In my tests this
+                // improved the
+                // ingestion performance fom 1.1m to 1.55m values per second on average
+                synchronized (this) {
                     final CsvToEntryTransformer csvToEntryTransformer = new CsvToEntryTransformer(queue, settings);
                     try (InputStream in = file.getInputStream()) {
                         csvToEntryTransformer.readCSV(in);
@@ -49,6 +55,7 @@ public class CsvUploadHandler implements PropertyKeys, DisposableBean {
                         LOGGER.error("csv ingestion failed", e);
                     }
                 }
+            }
         } catch (final RuntimeException e) {
             FileUtils.deleteSilently(tmpFiles);
             throw e;