From: David Ahern <dsah...@kernel.org> Date: Wed, 20 Mar 2019 09:18:59 -0700
> From: David Ahern <dsah...@gmail.com> > > fib_trie implementation calls synchronize_rcu when a certain amount of > pages are dirty from freed entries. The number of pages was determined > experimentally in 2009 (commit c3059477fce2d). > > At the current setting, synchronize_rcu is called often -- 51 times in a > second in one test with an average of an 8 msec delay adding a fib entry. > The total impact is a lot of slow down modifying the fib. This is seen > in the output of 'time' - the difference between real time and sys+user. > For example, using 720,022 single path routes and 'ip -batch'[1]: > > $ time ./ip -batch ipv4/routes-1-hops > real 0m14.214s > user 0m2.513s > sys 0m6.783s > > So roughly 35% of the actual time to install the routes is from the ip > command getting scheduled out, most notably due to synchronize_rcu (this > is observed using 'perf sched timehist'). > > This patch makes the amount of dirty memory configurable between 64k where > the synchronize_rcu is called often (small, low end systems that are memory > sensitive) to 64M where synchronize_rcu is called rarely during a large > FIB change (for high end systems with lots of memory). The default is 512kB > which corresponds to the current setting of 128 pages with a 4kB page size. > > As an example, at 16MB the worst interval shows 4 calls to synchronize_rcu > in a second blocking for up to 30 msec in a single instance, and a total > of almost 100 msec across the 4 calls in the second. The trade off is > allowing FIB entries to consume more memory in a given time window but > but with much better fib insertion rates (~30% increase in prefixes/sec). > With this patch and net.ipv4.fib_sync_mem set to 16MB, the same batch > file runs in: > > $ time ./ip -batch ipv4/routes-1-hops > real 0m9.692s > user 0m2.491s > sys 0m6.769s > > So the dead time is reduced to about 1/2 second or <5% of the real time. > > [1] 'ip' modified to not request ACK messages which improves route > insertion times by about 20% > > Signed-off-by: David Ahern <dsah...@gmail.com> Looks nice, applied, thanks David.
