Messages in this thread | | | Date | Wed, 5 Aug 2009 12:39:06 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: [PATCH 3/12] ksm: pages_unshared and pages_volatile |
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On Tue, 4 Aug 2009, Andrew Morton wrote: > On Mon, 3 Aug 2009 13:11:53 +0100 (BST) > Hugh Dickins <hugh.dickins@tiscali.co.uk> wrote: > > > pages_volatile is harder to define. It includes those pages changing > > too fast to get into the unstable tree, but also whatever other edge > > conditions prevent a page getting into the trees: a high value may > > deserve investigation. Don't try to calculate it from the various > > conditions: it's the total of rmap_items less those accounted for. ... > > static inline struct rmap_item *alloc_rmap_item(void) > > { > > - return kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL); > > + struct rmap_item *rmap_item; > > + > > + rmap_item = kmem_cache_zalloc(rmap_item_cache, GFP_KERNEL); > > + if (rmap_item) > > + ksm_rmap_items++; > > + return rmap_item; > > } > > ksm_rmap_items was already available via /proc/slabinfo. I guess that > wasn't a particularly nice user interface ;)
procfs is not a nice interface for sysfs to be reading when it's asked to show pages_volatile!
And not even always available, I think: SLOB wouldn't be able to report the number of objects of any particular type, SLUB would need slub_nomerge (or a debug flag) to keep the kmem_cache separate; and even SLAB would have to assemble numbers from different cpus and queues, I guess. Easier and more reliable for KSM to do its own thing here.
I do agree that slabinfo or slqbinfo or /proc/slabinfo is a good enough interface for checking up on the number of rmap_items in use; it's what I was using most of the time. I did once fleetingly wonder whether to show the count of rmap_items under /sys/kernel/mm/ksm, but rejected it as not interesting enough to deserve more than what slabinfo tells.
But here the rmap_item count is being used to deduce something more interesting (though more obscure), and I don't want SL?B divergences to perturb the resulting number more than it is already. I did start out without the rmap_items count, incrementing and decrementing pages_volatile all over the place; but soon abandoned that in favour of the difference calculation.
But I think I'm taking you more seriously than you intended, sorry for my humourlessness!
Hugh
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