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authorDavid Ahern <dsahern@gmail.com>2019-03-20 19:18:59 +0300
committerDavid S. Miller <davem@davemloft.net>2019-03-21 23:29:53 +0300
commit9ab948a91b2c2abc8e82845c0e61f4b1683e3a4f (patch)
treed096d05c00760a38c1d91b6f5be5bd91fc28a445 /Documentation
parent12132768dc4a79be65af75ac6262117d0adf93f3 (diff)
downloadlinux-9ab948a91b2c2abc8e82845c0e61f4b1683e3a4f.tar.xz
ipv4: Allow amount of dirty memory from fib resizing to be controllable
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 <dsahern@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/networking/ip-sysctl.txt5
1 files changed, 5 insertions, 0 deletions
diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index bd029fc55ccb..5eedc6941ce5 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -81,6 +81,11 @@ fib_multipath_hash_policy - INTEGER
0 - Layer 3
1 - Layer 4
+fib_sync_mem - UNSIGNED INTEGER
+ Amount of dirty memory from fib entries that can be backlogged before
+ synchronize_rcu is forced.
+ Default: 512kB Minimum: 64kB Maximum: 64MB
+
ip_forward_update_priority - INTEGER
Whether to update SKB priority from "TOS" field in IPv4 header after it
is forwarded. The new SKB priority is mapped from TOS field value