Chris Snook wrote: > Al Boldi wrote: > > IMHO, what everybody agrees on, is that swap-prefetch has a positive > > effect in some cases, and nobody can prove an adverse effect (excluding > > power consumption). The reason for this positive effect is also crystal > > clear: It prefetches from swap on idle into free memory, ie: it doesn't > > force anybody out, and they are the first to be dropped without further > > swap-out, which sounds really smart. > > > > Conclusion: Either prove swap-prefetch is broken, or get this merged > > quick. > > If you can't prove why it helps and doesn't hurt, then it's a hack, by > definition.
Ok, slow down: swap-prefetch isn't a hack. It's a kernel-thread that adds swap-prefetch functionality to the kernel. > With swap prefetch, we're only optimizing the case when the box isn't > loaded and there's RAM free, but we're not optimizing the case when the > box is heavily loaded and we need for RAM to be free. Exactly, swap-prefetch is very specific, and that's why it's so successful: It does one thing, and it does that very well. > I'm inclined to view swap prefetch as a successful scientific experiment, > and use that data to inform a more reasoned engineering effort. If we can > design something intelligent which happens to behave more or less like > swap prefetch does under the circumstances where swap prefetch helps, and > does something else smart under the circumstances where swap prefetch > makes no discernable difference, it'll be a much bigger improvement. Well, a swapless OS would really be the ultimate, but that's another thread entirely (see thread: '[RFC] VM: I have a dream...') Don't mistake swap-prefetch as trying to additionally fix swap-in slowdown, and if it did, then that would be a hack, but it doesn't. Instead, understand that swap-prefetch is viable even if all swapper issues have been solved, because swapping implies pages being swapped in when needed, and swap-prefetch smartly uses idle time to do so. > Because we cannot prove why the existing patch helps, we cannot say what > impact it will have when things like virtualization and solid state drives > radically change the coefficients of the equation we have not solved. > Providing a sysctl to turn off a misbehaving feature is a poor substitute > for doing it right the first time, and leaving it off by default will > ensure that it only gets used by the handful of people who know enough to > rebuild with the patch anyway. But we do know why it helps: a proc eats memory, then page-cache, then swaps others out, and then dies to free its memory, and now swap-prefetch comes in if the system is idle. Sounds really smart. While many people may definitely benefit, others may just want to turn it off. No harm done. Thanks! -- Al - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
