28/10/2020 11:42, Nithin Dabilpuram: > On Wed, Oct 28, 2020 at 10:24:01AM +0000, Van Haaren, Harry wrote: > > From: Thomas Monjalon > > > 28/10/2020 10:30, Nithin Dabilpuram: > > > > From: Thomas Monjalon <tho...@monjalon.net> > > > > > > > > The node_mbuf_priv1 was stored in the deprecated mbuf field udata64. > > > > It is moved to a dynamic field in order to allow removal of udata64. > > > > > > > > Signed-off-by: Thomas Monjalon <tho...@monjalon.net> > > > > Signed-off-by: Nithin Dabilpuram <ndabilpu...@marvell.com> > > > [...] > > > > + IP4_LOOKUP_NODE_PRIV1_OFF(node->ctx) = > > > node_mbuf_priv1_dynfield_offset; > > > > > > That's interesting. > > > You copy the offset in the node context for better performance. > > > How much is it better than with global offset variable? > > > How much it decreases compared to a static mbuf field? > > > > Also interested in this topic, I'll offer the logical/theory point of view; > > > > With a static field, the offset into the mbuf can be encoded in the > > instruction > > stream, meaning there are no d-cache loads to identify particular dynamic > > field. > > > > With a static/global variable, the cache line where the value resides is > > presumably > > not hot in cache per burst (assuming an application that does significant > > work, so not > > in cache since last burst). Hence overhead estimate could be 1x cache line > > load per burst. > > > > With the data copied into the node, the offset is presumably on a hot cache > > line as the > > node is using other data-members of its context. As a result, perhaps a > > cold static cache > > line load is converted to a hot node-context line re-use. > > > > Real world overhead likely depends on A) does the application cache-trash > > enough to make > > the static/global line fall out of cache - causing perf degradation due to > > reload, and B) does > > the node->ctx still fit in the same number of lines as before if the value > > is copied there. > > Agreed, node->ctx is already referred to get other data (lpm pointer). So > referening another 4 bytes might even convert that to load pair which is at > no extra cost. > > Number's wise, > it decreases by ~1.4 % from static mbuf field to global offset variable > and it decreases by ~1% from static mbuf field to node context field > cached per process call
OK thanks for providing these numbers.
