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Meeting on Thursday, April 26, 2007 [message #4170] Wed, 25 April 2007 16:31 Go to previous message
Silvia Masciocchi is currently offline  Silvia Masciocchi
Messages: 162
Registered: May 2006
first-grade participant
From: *gsi.de
ALICE Computing Meeting
April 26, 2007


Preliminary agenda:

1. Origin of high failing rate of Alien jobs at GSI
How to proceed -
1.1 Detection of problem
1.2 Responsible from KP1 group to fix possible problems, if they are AliRoot related
1.3 What tools can we use to detect problems
(see informations below)

2. Alien SE at GSI
2.1 How to make it working? - Responsible?
2.2 Can we use it already for staging of the fraction of PDC06 data?

3. XRD copy
(see informations below)

==============================================================
VERY QUICK MINUTES (27.04.2007, Silvia)

We discussed only point 1.

a. currently only one GRID job at the time can run on the D-Grid machines, despite their 8GB memory (but only 2GB swap). Silvia will test some parallel running of AliRoot jobs executed in "stand-alone" mode instead of via AliEn (see how many jobs per machine can run, and when/where the memory limits are reached)
b. we decided to postpone further investigations about that fact that GRID jobs cannot be executed on the batch farm to when Kilian will be back at GSI
c. Marian and Silvia will investigate the memory consumption of AliRoot during a typical simulation+reconstruction job (using tools like valgrind). Possible problems (like memory leaks) will be reported to the software/ detector responsibles.

Below, please, find quite some material collected about the memory consumption of AliRoot GRID jobs and details of the failures happening when copying to the xrootd cluster.

No formal meeting will take place in the next 2 weeks. Information and results will be distributed via the forum and emails.

==============================================================
INFORMATION ABOUT THE GRID JOBS AT GSI

1 -- STATUS AS REPORTED BY MONALISA

1.1 Summary table for all centers: see
http://pcalimonitor.cern.ch:8889/reports/index.jsp (in daily, weekly, monthly summaries)

1.2 Integrated CPU time for ALICE jobs at GSI compared to Muenster, Catania and FKZ (the pledged resources of these centers are, respectively, 100, 132, 138 and 281 KSI2K) http://pcalimonitor.cern.ch:8889/display?SiteBase=NIHAM&err=0&im gsize=800x550&interval.max=0&interval.min=2628000000&log=0&a mp;a mp;a mp;a mp;a mp;a mp;a mp;a mp;a mp;page=jobResUsageSum_time_cpu&plot_series=Catania&plot_series= FZK&plot_series=GSI&plot_series=Muenster&submit_plot=Plot&am p;am p;am p;am p;am p;am p;am p;am p;am p;sum=0


2 -- POSSIBLE REASON 1: HIGH MEMORY CONSUMPTION

The high memory consumption by AliRoot is indicated as main reason for the low efficiency at GSI.
But:

2.1 The jobs executed at GSI do *NOT* consume a higher amount of memory compared to other ALICE GRID sites, see
Virtual:
http://pcalimonitor.cern.ch:8889/display?Nodes=max&SiteBase=Jyvaskyl a&err=0&imgsize=800x550&interval.max=0&interval.min=7884 000000&log=0&page=jobResUsageMax_virtualmem&plot_series=FZK& amp; amp; amp; amp; amp; amp; amp; amp; amp;plot_series=GSI&submit_plot=Plot&sum=0

Resident:
http://pcalimonitor.cern.ch:8889/display?Nodes=max&SiteBase=Jyvaskyl a&err=0&imgsize=800x550&interval.max=0&interval.min=7884 000000&log=0&page=jobResUsageMax_rss&plot_series=FZK&plo t_series=GSI&submit_plot=Plot&sum=0


2.2 Other ALICE GRID sites generally have machines with the same memory: at best 2GB/core (raw and swap) (information from Latchezar Betev and Jan-Fiete)

2.3 My (Silvia) experience from interactive tests (on lxial36 and lxb255) and from thousands of jobs executed on the LSF farm (batch queue) confirms the memory consumption reported by the others (this is high on an absolute scale, but not higher at GSI than other centers). The MC production jobs done on the batch farm (many with a configuration almost identical to the PDC06) over the last several months were always very successful (above 95%).
Latest numbers: Aliroot v4-05-08, PDC06 configuration file and macros, 10 events per job (max memory used during the job):
lxial36 (32 bit, 1GB mem, 2GB swap) sim: 460MB, rec 470MB
lxb255 (64 bit, 8GB mem, 2GB swap): sim 640MB, rec 600MB
It is known that the memory usage is slightly higher on 64 bit machines.
I will update the numbers for 100 events/job in short, in order to compare with the performance on the batch farm.

2 -- POSSIBLE REASON 2: GSI BANDWIDTH

[Extract from an email by Kilian, April 18, 2007]
... The I/O of the ALICE jobs (downloading input files from CERN, putting output back to CERN) rather fast outnumbers the bandwitdth possibilities at GSI. This is why we are trying to route the Grid job I/O via the "Suedhessennetz" where we have a seperate 100 Mb for Grid only use for test reasons. Here I am still facing a political problem, though, that T2 packages are not yet allowed to be routed via T1 centres to the T0 centre via the dedicated T1-T0 links. But this is already known to Latchezar and the other Grid experts and we will work it out. [...]

What is not known is for WHICH FRACTION of the failures this second reason is responsible.

==============================================================

PROBLEMS WITH COPY TO THE XROOTD CLUSTER AT GSI

Problems have been encountered and reported by Marian and Silvia when copying large amount of data to the xrootd cluster at GSI. The copy process fails at some point of the data transfer and the new copy of the file results corrupted and unrecoverable.
A few features of these failures:
- the problem affects about 50-70% of the files copied to the xrootd cluster
- it happens while WRITING to the xrootd clusters. Reading did not present problems so far
- it happens both while copying from GSI NFS disks (Silvia: MC production, both from machines on the batch farm and from /d/aliceXY disks), and from AliEn (Marian: TPC data from CERN)
- the failure is not reproducible: in repeated attempts to copy a file, the failure can re-appear or not, and usually at different times of the copy
- in Marian's example, copying TPC data from AliEn, the failure rate was about 50% when writing them to the xrootd cluster, but 0% when writing them to a local disk. See his script
/u/miranov/AliRoot6/HEAD0307/TPC/macros/alienDownload.C
(the 2 cases correspond, respectively, to the destinations:
TString gsipath="root://lxfs35.gsi.de:1094//alice/testtpc/raw2006/";
and
TString gsipath="/data.local2/miranov/raw2006/";).
- Marian also tried to repeat up to 5 times the copy from AliEn, but this did not improve sensibly the situation: some failure appeared all the time.

Horst suggested that the problem might be connected to poor network connections, which cause transfer delays. Extract from his email (April 24, 2007):
[...] xrootd is optimized for high performance transfers, and I guess that the time-outs in client server communication are very tough. As you possibly know our network is currently not always as stable as it should be. This could be improved with HW upgrades, but we don't have enough money for this now. I'm not an expert in xrootd internals, but if occasional network delays (~1 s) are responsible for your copy problems, repeating the copy command should help (of course, this is not acceptable, but maybe a hint to the root of the problem).[...]
(about the fact that repeating the copy does NOT solve the problem, see examples above).

This should be further investigated and proved. We should see if the acceptable timeout could be set to larger times, in xrootd (if this is the reason).
One more test which is pending is copying large amount of data (TPC, PDC06) to the D-Grid machines with xrootd deamon (waiting for AliEn to come back online).

[Updated on: Fri, 27 April 2007 15:03]

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