Messages in this thread | | | Date | Thu, 10 Nov 2005 15:13:29 -0800 (PST) | From | Christoph Lameter <> | Subject | Re: [RFC, PATCH] Slab counter troubles with swap prefetch? |
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On Fri, 11 Nov 2005, Con Kolivas wrote:
> > This patch splits the counter into the nr_local_slab which reflects > > slab pages allocated from the local zones (and this number is useful > > at least as a guidance for the VM) and the remotely allocated pages. > > How large a contribution is the remote slab size likely to be? Would this > information be useful to anyone potentially in future code besides swap > prefetch? The nature of prefetch is that this is only a fairly coarse measure > of how full the vm is with data we don't want to displace. Thus it is also > not important that it is very accurate.
The size of the remote cache depends on many factors. The application can influence that by setting memory policies.
> Unless the remote slab size can be a very large contribution, or having local
Yes it can be quite large. On some of my tests with applications these are 100%. This is typical if the application sets the policy in such a way that all allocations are off node or if the kernel has to allocate memory on a certain node for a device.
> and remote slab sizes is useful potentially to some other code I'm inclined > to say this is unnecessary. A simple comment saying something like "the > nr_slab estimation is artificially elevated by remote slab pages on numa, > however this contribution is not important to the accuracy of this > algorithm". Of course it is nice to be more accurate and if you think > worthwhile then we can do this - I'll be happy to be guided by your > judgement.
> As a side note I doubt any serious size numa hardware will ever be idle enough > by swap prefetch standards to even start prefetching swap pages. If you think > hardware of this sort is likely to benefit from swap prefetch then perhaps we > should look at relaxing the conditions under which prefetching occurs.
Small scale NUMA machines may benefit from swap prefetch but on larger machines people usually try to avoid swap altogether.
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