Messages in this thread | | | Date | Tue, 15 May 2001 00:13:23 -0600 | From | Richard Gooch <> | Subject | Re: Getting FS access events |
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Linus Torvalds writes: > > On Mon, 14 May 2001, Richard Gooch wrote: > > > > Is there some fundamental reason why a buffer cache can't ever be > > fast? > > Yes. > > Or rather, there is a fundamental reason why we must NEVER EVER look at > the buffer cache: it is not coherent with the page cache. > > And keeping it coherent would be _extremely_ expensive. How do we > know? Because we used to do that. Remember the small mindcraft > benchmark? Yup. Double copies all over the place, double lookups, double > everything. > > You could think: "oh, we only need to look up the buffer cache when we > create a new page cache mapping, so..". > > You'd be wrong. We'd need to go the other way too: every time we create a > new buffer cache entry, we'd need to make sure that it isn't mapped > somewhere in the page cache (impossible), or otherwise we'd do the wrong > thing sometimes (ie we might have two dirty copies, and we wouldn't know > _which_ one is valid etc). > > Aliasing is bad. Don't do it.
OK, this (combined with the other message) explains why we want to keep away from the buffer cache. Thanks.
> You know, the mark of intelligence is realizing when you're making > the same mistake over and over and over again, and not hitting your > head in the wall five hundred times before you understand that it's > not a clever thing to do.
But you didn't have to add this. Please note that I asked why not use the buffer cache. I didn't proclaim that it was the ideal solution. I did say what benefits it had, but I didn't assert that the benefits outweighed the disadvantages.
> Please show some intelligence.
Well, frankly, I think I have. Things are obvious when you know them already. Even if I'm ignorant, I'm not stupid!
Regards,
Richard.... Permanent: rgooch@atnf.csiro.au Current: rgooch@ras.ucalgary.ca - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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