Re: Linux memory zone reclaim

On Fri, Aug 3, 2012 at 4:30 PM, Josh Berkus <josh(at)agliodbs(dot)com> wrote:
> On 7/30/12 10:09 AM, Scott Marlowe wrote:
>> I think the zone_reclaim gets turned on with a high ratio. If the
>> inter node costs were the same, and the intranode costs dropped in
>> half, zone reclaim would likely get turned on at boot time.
>
> We've been seeing a major problem with zone_reclaim and Linux, in that
> Linux won't use the FS cache on the "distant" RAM *at all* if it thinks
> that RAM is distant enough. Thus, you get instances of seeing only half
> of RAM used for FS cache, even though the database is 5X larger than RAM.
>
> This is poor design on Linux's part, since even the distant RAM is
> faster than disk. For now, we've been disabling zone_reclaim entirely.

I haven't run into this, but we were running ubuntu 10.04 LTS. What
kernel were you running when this happened? I'd love to see a test
case on this, as it seems like a major regression if it's on newer
kernels, and we're looking at running 12.04 LTS soon on one of our
bigger machines.