On Fri, 1 Feb 2008, Alexander Motin wrote:
That was actually my second question. As there is only 512 items by default
and they are small in size I can easily preallocate them all on boot. But is
it a good way? Why UMA can't do just the same when I have created zone with
specified element size and maximum number of objects? What is the principal
difference?
Alexander,
I think we should drill down in the analysis a bit and see if we can figure
out what's going on with UMA. What UMA essentially does is ask the VM for
pages, and then pack objects into pages. It maintains some meta-data, and
depending on the relative sizes of objects and pages, it may store it in the
page or potentially elsewhere. Either way, it looks very much an array of
struct object. It has a few extra layers of wrapping in order to maintain
stats, per-CPU caches, object life cycle, etc. When INVARIANTS is turned off,
allocation from the per-CPU cache consists of pulling objects in and out of
one of two per-CPU queues. So I guess the question is: where are the cycles
going? Are we suffering excessive cache misses in managing the slabs? Are
you effectively "cycling through" objects rather than using a smaller set that
fits better in the cache? Is some bit of debugging enabled that shouldn't be,
perhaps due to a failure of ifdefs?
BTW, UMA does let you set the size of buckets, so you can try tuning the
bucket size. For starts, try setting the zone flag UMA_ZONE_MAXBUCKET.
It would be very helpful if you could try doing some analysis with hwpmc --
"high resolution profiling" is of increasingly limited utility with modern
CPUs, where even a high frequency timer won't run very often. It's also quite
subject to cycle events that align with other timers in the system.
Robert N M Watson
Computer Laboratory
University of Cambridge
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