On Tue, Jan 18, 2022 at 09:25:22AM +0100, Morten Brørup wrote: > > 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. > Thanks for all the explanation.
Reviewed-by: Bruce Richardson <bruce.richard...@intel.com>
