On Fri, 23 Oct 2009, Mel Gorman wrote: > > Hmm, is this really supposed to be added to __alloc_pages_high_priority()? > > By the patch description I was expecting kswapd to be woken up > > preemptively whenever the preferred zone is below ALLOC_WMARK_LOW and > > we're known to have just allocated at a higher order, not just when > > current was oom killed (when we should already be freeing a _lot_ of > > memory soon) or is doing a higher order allocation during direct reclaim. > > > > It was a somewhat arbitrary choice to have it trigger in the event high > priority allocations were happening frequently. > I don't quite understand, users of PF_MEMALLOC shouldn't be doing these higher order allocations and if ALLOC_NO_WATERMARKS is by way of the oom killer, we should be freeing a substantial amount of memory imminently when it exits that waking up kswapd would be irrelevant. > > If this is moved to the fastpath, why is this wake_all_kswapd() and not > > wakeup_kswapd(preferred_zone, order)? Do we need to kick kswapd in all > > zones even though they may be free just because preferred_zone is now > > below the watermark? > > > > It probably makes no difference as zones are checked for their watermarks > before any real work happens. However, even if this patch makes a difference, > I don't want to see it merged. At best, it is an extremely heavy-handed > hack which is why I asked for it to be tested in isolation. It shouldn't > be necessary at all because sort of pre-emptive waking of kswapd was never > necessary before. > Ahh, that makes a ton more sense: this particular patch is a debugging effort while the first two are candidates for 2.6.32 and -stable. Gotcha. > > Wouldn't it be better to do this on page_zone(page) instead of > > preferred_zone anyway? > > > > No. The preferred_zone is the zone we should be allocating from. If we > failed to allocate from it, it implies the watermarks are not being met > so we want to wake it. > Oops, I'm even more confused now :) I thought the existing wake_all_kswapd() in the slowpath was doing that and that this patch was waking them prematurely because it speculates that a subsequent high order allocation will fail unless memory is reclaimed. I thought we'd want to reclaim from the zone we just did a high order allocation from so that the fastpath could find the memory next time with ALLOC_WMARK_LOW. -- To unsubscribe from this list: send the line "unsubscribe kernel-testers" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
