On Mon, Oct 01, 2012 at 08:34:28AM -0700, H. Peter Anvin wrote: > On 09/29/2012 06:48 AM, Andrea Arcangeli wrote: > > > > There would be a small cache benefit here... but even then some first > > level caches are virtually indexed IIRC (always physically tagged to > > avoid the software to notice) and virtually indexed ones won't get any > > benefit. > > > > Not quite. The virtual indexing is limited to a few bits (e.g. three > bits on K8); the right way to deal with that is to color the zeropage, > both the regular one and the virtual one (the virtual one would circle > through all the colors repeatedly.) > > The cache difference, therefore, is *huge*. Kirill measured the cache benefit and it provided a 6% gain, not very huge but certainly significant. > It's a performance tradeoff, and it can, and should, be measured. I now measured the other side of the trade, by touching only one character every 4k page in the range to simulate a very seeking load, and doing so the physical huge zero page wins with a 600% margin, so if the cache benefit is huge for the virtual zero page, the TLB benefit is massive for the physical zero page. Overall I think picking the solution that risks to regress the least (also compared to current status of no zero page) is the safest. Thanks! Andrea -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
