topic Re: Missing dormqr equivalent in MKL Compact API for QR-based Batch Solve in Intel® oneAPI Math Kernel Library

Missing dormqr equivalent in MKL Compact API for QR-based Batch Solve

ivanp — Thu, 15 Jan 2026 10:53:25 GMT

I would like to solve a batch of (square) linear systems using the compact/interleaved QR factorization routine, mkl_?geqrf_compact. However, I am struggling to find the documented "compact" way to complete the linear system solve AX=B using the QR factorization. The matrices I'm interested in have a strange structure, meaning that the non-pivoting LU factorization is not applicable.

In standard LAPACK, I would use dormqr to compute Q^T B and then dtrsm for the back-substitution R X = (Q^T B).

Additionally, are there any technical white papers or articles (for instance, in Intel Parallel Universe Magazine) that provide a deep dive into the Compact/Interleaved solver suite?

Thank in advance for any advice.

Re: Missing dormqr equivalent in MKL Compact API for QR-based Batch Solve

ivanp — Sun, 12 Apr 2026 10:09:15 GMT

Is there any chance this could be part of the oneMKL 2026.0 release?

Essentially, what I'd need is similar to the ArmPL function,

armpl_dormqr_interleave_batch

or in MKL API, this would be

mkl_?ormqr_compact

Re: Missing dormqr equivalent in MKL Compact API for QR-based Batch Solve

Chao_Y_Intel — Wed, 22 Apr 2026 08:55:21 GMT

Hi @ivanp

Thanks for submitting this request. We’ve tracked this request in our internal systems so it can be reviewed as part of our regular planning process. Given other higher‑priority items, this feature has not been planned for near-term implementation yet.

In the meantime, if you have a Priority Support account, you may also submit questions through our Priority Support channel at https://supporttickets.intel.com/ for faster response on technical issues and product-related inquiries.

Regards,
Chao Y.

Intel MKL Support

Re: Missing dormqr equivalent in MKL Compact API for QR-based Batch Solve

ivanp — Thu, 02 Jul 2026 20:38:15 GMT

Meanwhile, I have generated my own implemention of the compact ?ormqr routine using Claude Code. The code can be found here: https://github.com/ivan-pi/cqr

I ran a benchmark on Sapphire Rapids for a batch of 10000 matrices across different sizes obtaining the following results:

~/cqr> ./build/bench_qr_compact 10000 3 QR solve throughput: compact batched (mkl_dgeqrf_compact -> cqr_mkl_dormqr_compact -> mkl_dtrsm_compact) vs per-matrix (LAPACKE_dgeqrf -> LAPACKE_dormqr -> cblas_dtrsm) matrices=10000 reps=3 simdlen=8 (AVX512) OpenMP threads=1 n | batched (s) Mmat/s | unbatched (s) Mmat/s | speedup | max fwd err -----+----------------------+----------------------+---------+------------- 10 | 0.0017 5.76 | 0.0167 0.60 | 9.60x | 1.78e-15 (rtol 2.2e-13) 20 | 0.0066 1.52 | 0.0409 0.24 | 6.22x | 2.44e-15 (rtol 4.4e-13) 30 | 0.0194 0.52 | 0.0647 0.15 | 3.34x | 3.11e-15 (rtol 6.7e-13) 40 | 0.0488 0.20 | 0.0980 0.10 | 2.01x | 3.55e-15 (rtol 8.9e-13) 50 | 0.0975 0.10 | 0.1838 0.05 | 1.88x | 3.33e-15 (rtol 1.1e-12) 60 | 0.1712 0.06 | 0.2544 0.04 | 1.49x | 4.44e-15 (rtol 1.3e-12) 80 | 0.4141 0.02 | 0.4821 0.02 | 1.16x | 5.55e-15 (rtol 1.8e-12) 100 | 0.8086 0.01 | 0.9542 0.01 | 1.18x | 4.55e-15 (rtol 2.2e-12) -----+----------------------+----------------------+---------+------------- geometric-mean speedup (batched vs unbatched) across sizes: 2.50x

To get an impression of the relative costs of dgeqrf_compact, dormqr_compact and dtrsm_compact, versus the non-compact variants, I have sampled the execution of the benchmark with perf (sampling at 999 Hz and last branch records for the callgraph). The generated flamegraph is shown below; the compact routines are shaded in pink:

The compact dormqr routine is a small fraction of the total samples. The non-compact version (LAPACKE_dormqr) accounted for 9.47 % of the samples, compared to 0.56 % for the compact version.

It is nice to see that the compact/batched approach pays off.

Re: Missing dormqr equivalent in MKL Compact API for QR-based Batch Solve

william1234 — Fri, 17 Jul 2026 10:25:35 GMT

As far as I know, ?geqrf_compact only provides the QR factorization. There isn't a documented compact equivalent of ?ormqr + ?trsm to complete the solve. In most cases, you'll need to use the standard LAPACK routines after factorization. Unfortunately, detailed documentation on the compact/interleaved APIs is also quite limited beyond the MKL reference manual.

Re: Missing dormqr equivalent in MKL Compact API for QR-based Batch Solve

ivanp — Fri, 17 Jul 2026 13:04:52 GMT

Hi @william1234

mkl_?trsm_compact is available in Intel MKL, but compact ?ormqr is indeed missing as discussed above. The documentation was sufficient for my needs. Most of the parameters match the the LAPACK routines, it is just the layout that is different (arrays contiguous along the batch direction) and the ?gepack/?geunpack helper functions that one has to learn to use. My next wish would be to have compact ?syrk; that would be useful in the context of a block Cholesky factorization.

I found some useful information about the batched approach here: https://icl.utk.edu/bblas/.
A presentation by Sarah Knepper (Intel) from a SC24 workshop is also linked therein: https://icl.utk.edu/bblas/sc24/files/3-EvolutionOfBatchedLinearAlgebraInOneMKL.pdf (PDF, 316 KB).

Pieter Pas from KU Leuven has created a C++ package for batched operations on small matrices: https://github.com/tttapa/batmat

Regards,
Ivan