ROOT IO workshop What relates to ATLAS

General Both CMS and Art pushing for parallelization at all levels. Not clear why as they are anyhow CPU bound and simple forking will spare them more than enough memory Art wants native byte ordering, C++14 support, lightweight, thread safe histogram object Alice now goes the way of ATLAS AOD flattening. Requests asynchronous de-serialization, thread safe hisotgrams and trees without locking.

ATLAS We want to ensure xAODs are WAN access efficient. as small as possible need smallest possible number of read requests have it all in root even if we can have it in our framework

Requests Old request – TTreeCache to be ON by default – coded but still in testing by David Smith. Compression – ESDs and AODs had custom compression algorithms in transient/persistent layer of IO. That is gone with xAODs. Size of some collections went up a lot. Need to recuperate at least a part. – Two main custom compressions: stripping bits (reduces precision) and covariance matrix compression – ROOT provides float16 and double32 types but that’s not sufficient and is very difficult to use – We requested new methods for creating branches, where one could specify precision desired. – ROOT team agreed.

Requests Optimized basket size – Better algorithm written 7 years ago. Tested on ATLAS data (not xAOD) and CMS. – Reduces total number baskets in file, improves occupancy – With less reads needed improves IO rates, slightly reduces data size, memory consumption. – Issues explained to David Smith. He will work on it.

Error handling We have all seen cases where ROOT does not handle well errors on accessing xrootd files, specially when using TChain. – ROOT team stance – report as many bug reports as needed. – Will not throw exceptions, but will correctly forward error codes. – Not a trivial thing to find/reproduce a problem as it can happen at any moment of ROOT/xrootd plugin interaction.

AsynchronousPrefetch Our current xAOD IO libraries by default use 20MB of TTC, but no AsyncPrefetch Change in how implemented (compared to what explained at last CHEP). Now equivalent as adding a stream. This could mean up to factor 2 read speed gain. We must extensively test this.

Monitoring xAOD IO Discussed between Attila, Paul, Ilija All the data accesses will be monitored (both Athena and straight ROOT) Initial list of variables agreed upon Initial code collecting the information should be checked in today Info will be sent to the collection system Ilija will provide collecting and mining part, now coding different variants (MongoDB, GAE, Oracle). When it proves itself in ATLAS, ROOT people would really like to have the system monitor as large part of all the ROOT jobs as possible. Quite different set of variables to collect. ROOT team will try to find someone from CERN IT to instrument ROOT.