Re: [RFC PATCH] Prio_heap: heap_remove(), heap_maximum(), heap_replace() and heap_cherrypick()

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Balbir Singh (balbir@linux.vnet.ibm.com) wrote:
> * Mathieu Desnoyers <mathieu.desnoyers@efficios.com> [2010-10-06 14:03:23]:
> 
> > These added interfaces lets prio_heap users lookup the top of heap item without
> > performing any insertion, perform removal of the topmost heap entry, and also
> > replacement of topmost heap entry. This is useful if one need to use the result
> > of the lookup to determine if the current maximum should simply be removed or if
> > it should be replaced.
> > 
> > This is used by the Generic Ring Buffer to perform timestamp-based fusion-merge
> > of per-cpu buffer records into a single stream.
> >
> > Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> > Cc: Paul Menage <menage@google.com>
> > Cc: David Rientjes <rientjes@google.com>
> > Cc: Nick Piggin <nickpiggin@yahoo.com.au>
> > Cc: Balbir Singh <balbir@in.ibm.com>
> > Cc: Cedric Le Goater <clg@fr.ibm.com>
> > Cc: "Eric W. Biederman" <ebiederm@xmission.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > ---
> 
> The patchset looks good to me, one comment on cherry picking below
> 
> Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>

Thanks!

[...]
> > +/**
> > + * heap_cherrypick - remove a given element from the heap
> > + * @heap: the heap to be operated on
> > + * @p: the element
> > + *
> > + * Remove the given element from the heap. Return the element if present, else
> > + * return NULL. This algorithm has a complexity of O(n), which is higher than
> > + * O(log(n)) provided by the rest of this API.
> > + */
> > +extern void *heap_cherrypick(struct ptr_heap *heap, void *p);
> 
> One way to reduce cherrypick'ing in O(n) if n is large, is to sort the
> entire heap using heapify() and then doing a binary search. The cost
> of sorting is high, so it depends on how often and in what phases we
> cherry pick.

I need this to implement the ring buffer iterator "reset" primitive, which is
very infrequently called. So for my use-case, I really prefer to keep very fast
add/remove/get max (as implemented currently) without sorting the rest of the
heap. What you propose looks more like a balanced binary tree, which we already
have in the kernel, but clearly adds some overhead by requiring to keep the
whole tree sorted.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com