> From: Bruce Richardson [mailto:bruce.richard...@intel.com] > Sent: Monday, 17 January 2022 18.35 > > On Fri, Jan 14, 2022 at 05:36:50PM +0100, Morten Brørup wrote: > > A flush threshold for the mempool cache was introduced in DPDK > version > > 1.3, but rte_mempool_do_generic_get() was not completely updated back > > then, and some inefficiencies were introduced. > > > > This patch fixes the following in rte_mempool_do_generic_get(): > > > > 1. The code that initially screens the cache request was not updated > > with the change in DPDK version 1.3. > > The initial screening compared the request length to the cache size, > > which was correct before, but became irrelevant with the introduction > of > > the flush threshold. E.g. the cache can hold up to flushthresh > objects, > > which is more than its size, so some requests were not served from > the > > cache, even though they could be. > > The initial screening has now been corrected to match the initial > > screening in rte_mempool_do_generic_put(), which verifies that a > cache > > is present, and that the length of the request does not overflow the > > memory allocated for the cache. > > > > 2. The function is a helper for rte_mempool_generic_get(), so it must > > behave according to the description of that function. > > Specifically, objects must first be returned from the cache, > > subsequently from the ring. > > After the change in DPDK version 1.3, this was not the behavior when > > the request was partially satisfied from the cache; instead, the > objects > > from the ring were returned ahead of the objects from the cache. This > is > > bad for CPUs with a small L1 cache, which benefit from having the hot > > objects first in the returned array. (This is also the reason why > > the function returns the objects in reverse order.) > > Now, all code paths first return objects from the cache, subsequently > > from the ring. > > > > 3. If the cache could not be backfilled, the function would attempt > > to get all the requested objects from the ring (instead of only the > > number of requested objects minus the objects available in the ring), > > and the function would fail if that failed. > > Now, the first part of the request is always satisfied from the > cache, > > and if the subsequent backfilling of the cache from the ring fails, > only > > the remaining requested objects are retrieved from the ring. > > > > 4. The code flow for satisfying the request from the cache was > slightly > > inefficient: > > The likely code path where the objects are simply served from the > cache > > was treated as unlikely. Now it is treated as likely. > > And in the code path where the cache was backfilled first, numbers > were > > added and subtracted from the cache length; now this code path simply > > sets the cache length to its final value. > > > > 5. Some comments were not correct anymore. > > The comments have been updated. > > Most importanly, the description of the succesful return value was > > inaccurate. Success only returns 0, not >= 0. > > > > Signed-off-by: Morten Brørup <m...@smartsharesystems.com> > > --- > > I am a little uncertain about the reversing of the copies taking things > out > of the mempool - for machines where we are not that cache constrainted > will > we lose out in possible optimizations where the compiler optimizes the > copy > loop as a memcpy?
The objects are also returned in reverse order in the code it replaces, so this behavior is not introduced by this patch; I only describe the reason for it. I floated a previous patch, in which the objects were returned in order, but Jerin argued [1] that we should keep it the way it was, unless I could show a performance improvement. So I retracted that patch to split it up in two independent patches instead. This patch for get(), and [3] for put(). While experimenting using rte_memcpy() for these, I couldn't achieve a performance boost - quite the opposite. So I gave up on it. Reviewing the x86 variant of rte_memcpy() [2] makes me think that it is inefficient for copying small bulks of pointers, especially when n is unknown at compile time, and its code path goes through a great deal of branches. > > Otherwise the logic all looks correct to me. > > /Bruce [1]: http://inbox.dpdk.org/dev/calbae1ojcswxufanlwg5y-tnpkfhvvkq8sj3jpboo7obgeb...@mail.gmail.com/ [2]: http://code.dpdk.org/dpdk/latest/source/lib/eal/x86/include/rte_memcpy.h [3]: http://inbox.dpdk.org/dev/20220117115231.8060-1...@smartsharesystems.com/
