Disclaimer

The content of this blog is my personal opinion only. Although I am an employee - currently of Imagination Technologies's MIPS group, in the past of other companies such as Intellectual Ventures, Intel, AMD, Motorola, and Gould - I reveal this only so that the reader may account for any possible bias I may have towards my employer's products. The statements I make here in no way represent my employer's position, nor am I authorized to speak on behalf of my employer. In fact, this posting may not even represent my personal opinion, since occasionally I play devil's advocate.

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Sunday, June 21, 2009

Blogging from ISCA: WDDD: Emmett Witchel: Mondriaan-like

Wrote paper: lots of people need metadata. Mondriaan. Pixie dust.

Mondriaan is a general solution, but must be engineered to fit a specific problem.

Mukti-paper projects. Emphasize novelty. Features disappear because they didn't work; lack of space; lack of novelty.

Mondriaan gave metadata for each 32-bit word. No alignment restrictions. No warts.

MMP hardware: like page table. Perm Table Base. Perm Table. Protection Lookaside Buffer.

Mondriaan tradeoffs:

Space:

PLB refill costs: minimize refills / maximize reach.

SW overheads writing table entries: infrequent updates and/or simple table updates.

Mondriaan stored permissions data, but can handle arbitrary metadata.

Talk is something like a design rationale for Mondriaan. 2 bit permissions for each word. (Byte?) Run length encoded. Zones <= 4 for 16 words. Fallback to pointer to a bitmap entry.

(AFG: I'm not interested in zones. Bad security model. But the metadata mechanisms may be interesting.)

RLE made writes slow.

Mondrix wrote permissions table entries a lot. RLE made it slow. Fell back to using bitmap entries. Coarser grained protection. Page granularity.

Emmett walks through examples of other work using Mondriaan-like metadata.

Colorama. Luis Ceze HPCA 2007. Explicitly uses Mondriaan. Colors data structures.
Measured dynamic memory allocations (malloc) every 1,900 instructions.

Loki. OSDI 2008. All data has 32 bit security tag. Tags per page, tags per word.

Concusion:

Balance space, PLB reach, write overhead.

PLB reach neglected.

Measuring write overhead maybe too hard for research project: suggests counting, multiplying by a cost estimate.

Audience Q: Dave Christie. PLB coherency? A: Mondrix did not deal with.

Dave Christie of AMD (also of my high school) asked about single threaded coherency and multithreaded. It sounds like he has some familiarity with the issue.
I think that it is also interesting that David chose this talk to attend.

Colorama: Gabriel Loh, Luis Ceze, Emmett: not totally practical, but interesting. Luis: had some deadlocks, might need TM support as well.

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