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P latform stability and track-fit problems

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Platform stability and track-fit problems

M. Moulson, T. Spadaro, P. Valente

Tracking Meeting, 18 Jul 2001

- Test of DBV-10 on SunOS and AIX:
- Input: 1000 raw events
- Output in ksl stream:
- AIX: 11 events, incl. 3 not found on SunOS
- SunOS: 10 events, incl. 2 not found on AIX
- AIX or SunOS: 13 events
- Mostly KSTAG (one INTERTAG)
- 2 events found on both platforms with different length

- General caveat (?’s):
- Parameter space is huge; this is a quick survey
- Most tests done on very small (single-event) samples
- Tests should be done methodically once direction is clear

- Differences appear at track-fit level
- Reconstruction through PR identical
- including DTCE(1), DHRE(1) banks

- Differences appear in DBV-9
- Test DBV’s 7, 8, 9, 10 on a single event (?)
- Reconstruction in general different in each version
- Same on AIX and SunOS platforms for DBV-7, 8

- CVS history: changes in DBV-9
- dconvr:Spatial resolutions from data
- vtxfin:Various small bug fixes

- Suspect effect from new parameterization of hit resolution

- Cannot eliminate effect just by switching off algorithms
- Kink finding, track joining, M.S., hit add./rej., etc.
- Hit flipping not switchable
- Fine t-s relations a possible exception
- Known changes correspond to onset of differences (?)
- Provide a plausible mechanism for effect

- Cannot eliminate effect by disabling code-optimization

Input: 1000 raw events from run 18805

Table summarizes differences in ksl stream

DFITER

Get space points from track pars. (q)

DFTRAC

Time-space conversion

i < 1

DFBCOR

DFDRV

Get residuals, c2, V = dc2/dq

c2 > c2(old)

LEQU64

Vdq = q; q = q + dq

Dc2 < cut

CONVERGED

Max iter?

FAILED

- DFITER called at various points
- at start of event
- after each hit flipped in DFLIP
- after DFMUSC, DFDEDX, etc.
- at end of event

- Max iterations in a single call: 8
- On failure, convergence criterion relaxed and called again
- up to 15 times per track from most places
- up to 15 times more for dE/dx and at end of event

- Most tracks reconstructed differently show convergence problems

Track

- At first call to DFITER, parameters different by 10-5-10-6
- Inside DFITER, after LEQU64, difference increases to 10-5-10-4
- Differences accumulate with each call to DFITER
- Eventually jump bins in fine t-s
- Differences in %

- Problem exacerbated when convergence difficult
- Most critical track parameter: z
- Can diverge by tens of cm, esp. in DFLIP

Tries

DFITER

End Tries

Hits

Tries

DFITER

DFLIP

End Tries

End Hits

Other Alg

- Why do we see differences at 10-5 level at input to DFITER?
- In principle possible to have exact agreement between platforms for single calculation
- In practice depdends on optimization, autopromotion, rounding modes
- E.g., AIX: our standard compiler flags do not round in single precision but autopromote single to double

- Fair amount of code before this point; numerical errors accumulate rapidly

- Part of solution will involve:
- Tuning compilation parameters
- Promoting key parts of track fit to double precision
- NB: Matrix inversion already in double, looks OK
- Worst case: V-1V = 1 to within < 10-12 (diag), 10-9 (off diag)

- 1. Residuals and drift distances not updated if hit not flipped
- Basis for choice of next hit to flip
- Always assume previous hit was flipped

- 2. Failed DFITER calls count against max. retries
- Looser convergence if lots of hits to flip (lots of failures)
- Fewer calls to DFITER allowed
- If a hit won’t flip, do we want to retry?

- 3. Criterion for keeping flip: c2 < input c2
- Flip is kept even if c2 worse than best so far

- 4. More subjective issues
- No use of information on z-progression?

DFLIP

Get hits to flip

Sort; pick worst

Store track pars.

Flip hit

DFITER (15 times)

c2 < input

Restore track pars.

Return

- More study of the problem
- More sensible calling strategy for DFITER
- Small fixes to DFLIP (for now)
- Use l/2 instead of sampling for small drift distances
- Double precision at key points
- Compiler flags to consistently handle numerical inaccuracy
- Smoothing of t-s relations, resolution curves
- Evaluate efficacy of changes based on:
- parameter resolution, track c2, L/R resolution accuracy, track splitting, machine stability

- While monitoring traditional quantities:
- efficiency, hit efficiency, purity, CPU time

August downtime will be only point in near future when large-scale reprocessing possible