Re: [PATCH 0/6] Per-processor private data areas for i386

From: Pavel Machek
Date: Wed Sep 27 2006 - 15:50:06 EST


> [ Changes since previous post:
> - roll a new set of patches with all updates, based on 2.6.18-mm1 ]
> Implement per-processor data areas for i386.
> This patch implements per-processor data areas by using %gs as the
> base segment of the per-processor memory. This has two principle
> advantages:
> - It allows very simple direct access to per-processor data by
> effectively using an effective address of the form %gs:offset, where
> offset is the offset into struct i386_pda. These sequences are faster
> and smaller than the current mechanism using current_thread_info().
> - It also allows per-CPU data to be allocated as each CPU is brought
> up, rather than statically allocating it based on the maximum number
> of CPUs which could be brought up.
> Performance:
> I've done some simple performance tests on an Intel Core Duo running
> at 1GHz (to emphisize any performance delta). The results for the
> lmbench null syscall latency test, which should show the most negative
> effect from this change, show a ~9ns decline (.237uS -> .245uS).
> This corresponds to around 9 CPU cycles, and correlates well with
> the addition of the push/load/pop %gs into the hot path.

So we have 4% slowdown...

> I have not yet measured the effect on other typees of processor or
> more complex syscalls (though I would expect the push/pop overhead
> would be drowned by longer times spent in the kernel, and mitigated by
> actual use of the PDA).
> The size improvements on the kernel text are nice as well:
> 2889361 -> 2883936 = 5425 bytes saved

...and 0.2% smaller kernel. I guess you should demonstrate speedup at
complex syscalls before wedecide it is worth it...?

Thanks for all the (sleeping) penguins.
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