Messages in this thread | | | Date | Tue, 25 May 2004 23:48:17 +0200 | From | Ingo Molnar <> | Subject | Re: 4g/4g for 2.6.6 |
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* Andrew Morton <akpm@osdl.org> wrote:
> > Btw, you're right about the VMAs. Looking through customer > > stuff a bit more the more common issues are low memory being > > eaten by dentry / inode cache - which you can't always reclaim > > due to files being open, and don't always _want_ to reclaim > > because that could well be a bigger performance hit than the > > 4:4 split. > > I did some testing a year or two back with the normal zone wound down > to a few hundred megs - filesytem benchmarks were *severely* impacted > by the increased turnover rate of fs metadata pagecache and VFS > caches. I forget the details, but it was "wow".
and it's not only the normal workloads we know about - people really do lots of weird stuff with Linux (and we are happy that they use Linux and that Linux keeps chugging along), and people seem to prefer a 10% slowdown to a box that locks up or -ENOMEM's. I'm not trying to insert any unjustified fear, 3:1 can be ok with lots of RAM, but it's _clearly_ wishful thinking that 32 GB x86 will be OK with just 600 MB of lowmem. 600 MB of lowmem means a 1:80 lowmem to RAM ratio, which is insane. Yes, it will be OK with select workloads and applications. With 4:4 it's 3.4 GB lowmem and the ratio is down to a much saner 1:9, and boxes pushed against the wall keep up better.
(but i wont attempt to convince Andrea - and i'm not at all unhappy that he is trying to fix 3:1 to be more usable on big boxes because those efforts also decrease the RAM footprint of the UP kernel, which is another sensitive area. These efforts also help sane 64-bit architectures, so it's a win-win situation even considering our disagreement wrt. 4:4.)
> I'm suspecting we'll end up needing mempools (or something) of 1- and > 2-order pages to support large-frame networking. I'm surprised there > isn't more pressure to do something about this. Maybe people are > increasing min_free_kbytes.
hm, 1.5K pretty much seems to be the standard. Plus large frames can be scatter-gathered via fragmented skbs. Seldom is there a need for a large skb to be linear.
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