FYI, I'm finding other people on the net via Google who have seen this same problem when they went to 2.6.9-22. Downgrading to 2.6.9-11 seems to be fixing the problems for everyone else.
I'll open a ticket and see what happens. Don Don MacAskill wrote:
Tom Sightler wrote:On Mon, 2006-03-06 at 16:27 -0800, Don MacAskill wrote:We have an 8-way RHEL4 AS system (4 dual-core Opterons) with no swap (because swapping is still broken for DB loads. It swaps the DB out in preference of disk cache)....We're obviously not starving for RAM (3.4GB in disk cache), and we don't have any swap enabled, so what on earth is it doing and why is it pegging my CPU?I think these two statements together basically outline your problem. You assume that when swap is enabled and the systems starts swapping, that the system is broken. Then, you disable swap to fix the "brokeness" and wonder why kswapd uses lots of CPU. Well, it's kswapd's job to monitor free memory and make sure memory pages are free. Your system has 40MB free (cache usage still counts as used memory, even if you don't want to count it), which is probably low enough that kswap thinks it needs to free up some memory. It's busily scanning memory looking for stuff to swap out except, hey, what's this, there's no swap, so it's a wasted effort but that won't keep it from trying again and again.There was a big discussion about this problem on taroon-list a few years ago. We all spent quite a bit of time trying to tune the swap parameters and couldn't do it. Turning of swap fixed the problem 100%.Fact: There was major brokenness under DB loads with swap enabled. It *is* broken.Fact: As soon as we disabled swap, our DB boxes saw a 2X boost in performance.I've been running RHEL4 on this box for awhile, ran into a similar problem, played with the tuneables, finally disabled swap, my problems went away. Figured it would have been fixed in RHEL4, but no such luck.Recently, though, I updated to U2. I *think* (I can't be sure, alas, since I got busy) this new problem surfaced. Seems like I'm stuck - swap on, performance is bad. Swap off, I lose one of my CPU cores.If your system is swapping instead of reclaiming cache, don't just remove all swap, tune the parameters that control the behavior, namely swappiness and vfs_cache_pressure. Have you already attempted tuning with vfs_cache_pressure and not been successful?Yes, I've tried tuning all of the tuneables I can find. No real difference. I definitely played with swappiness, but I can't recall if I played with vfs_cache_pressure. I probably did, but it's possible I overlooked it - I'll look.I didn't just turn swap off as a first effort. It was a last resort - and I wasn't alone.Don -- nahant-list mailing list nahant-list redhat com https://www.redhat.com/mailman/listinfo/nahant-list