The route cache is a Linux kernel component enabling route lookups to be faster by caching the results in some table and checking it before issuing a regular lookup in the route tables. When using Linux as a router, the inefficiency of the route cache can hinder the performance of your box.
The documentation on this component is scarce and it is difficult to find up-to-date bits on how the route cache works and how to tune it. The book Understanding Linux Network Internals from O’Reilly is an exception and contains valuable information on how the route cache works. Even if the book is targeted at 2.6.12, the part on the route cache is still quite accurate. Unfortunately, it fails to provide appropriate tips on how to monitor and tune the route cache.
Overview of the route cache subsystem#
To handle an incoming or outgoing IP datagram, the kernel needs to issue a lookup in the route tables. While it seems to be quite trivial, several questions need to be answered:
- Does the source address and the destination address appear to be valid?
- Is the source address a martian address?3
- Is the destination address mine or should I forward the packet?
- Which route table should I use?
- Does the destination address match this route? And this one?
- Can I currently contact the gateway that I should use?
These checks can be a bit time consuming, even for small route
tables. To avoid them for each packets, Linux maintains a route
cache which is queried before doing a regular lookup and updated
after each one. You can dump it with
ip -s route show cache:
$ ip -s route show cache 198.51.100.7 from 203.0.113.2 via 192.0.2.1 dev eth1 cache used 7 age 2sec ipid 0x1fce rtt 131ms rttvar 45ms cwnd 10 198.51.100.17 from 203.0.113.15 via 192.0.2.1 dev eth1 cache used 3 age 0sec ipid 0xc3bd local 192.0.2.18 from 203.0.113.15 dev lo src 192.0.2.18 cache <local> used 154 age 1sec iif eth0
Here are two examples:
- If Linux receives a packet from 203.0.113.15 to 198.51.100.17, it will find this flow in the route cache. Therefore, it already knows that it should forward the packet to 192.0.2.1. No checks needed.
- If it receives a packet from from 203.0.113.16 to 198.51.100.17, there is no appropriate entry in the route cache and therefore, the system will have to look at the route tables. It is likely to use the exact same entry than if the source was 203.0.113.15 but maybe there is some policy routing requesting the use of a special route table or 203.0.113.16 is a local address and the packet will therefore be classified as martian.
Once an entry has been added to the route cache, there are several ways to remove it. Most entries are removed by the garbage collector which will scan the route cache and remove invalid and older entries. It will be triggered when the route cache is full or at regular interval, once a certain threshold has been met.
There are several values you can tune. Most of them are available in
rhash_entriesis the size of the hash table.5 If you don’t specify it on the kernel command line, it is computed dynamically based on the memory available on your system. You can view its value by looking at something like
IP route cache hash table entries: 262144 (order: 9, 2097152 bytes)in the kernel logs.
net.ipv4.route.max_sizeis the maximum number of entries in the route cache. Except under exceptional circumstances, this value is never exceeded.
net.ipv4.route.gc_elasticityis the target average length of a chain in the route cache. The garbage collector will work harder if this value is exceeded. This means that if you multiply this value by the value of
rhash_entries, you will get the target average number of entries in the route cache.
net.ipv4.route.gc_min_interval_msis the minimum delay between two runs of the garbage collector, except when the cache is full. The default value should be fine.
net.ipv4.route.gc_threshis a threshold triggering the garbage collector every
net.ipv4.route.gc_timeoutis the base value to determine if an entry is old enough to be removed or not. Whatever its value, the garbage collector will attempt to remove the same number of entries. However, this value could potentially influence its efficiency. See below for more details on this.
You may find documentation about these obsolete sysctl values:
net.ipv4.route.secret_intervalhas been removed in Linux 2.6.35; it was used to trigger an asynchronous flush at fixed interval to avoid to fill the cache.
net.ipv4.route.gc_intervalhas been removed in Linux 2.6.38. It is still present until Linux 3.2 but has no effect. It was used to trigger an asynchronous cleanup of the route cache. The garbage collector is now considered efficient enough for the job.
net.ipv4.route.gc_interval is back for Linux 3.2. It is still needed to avoid exhausting the
neighbour cache because it allows one to cleanup the cache
periodically and not only above a given threshold. Keep it to its
default value of 60.
Statistics & monitoring#
Linux maintains some statistics about the use of the route
cache. You can find them in
/proc/net/stat/rt_cache. The command
lnstat can print them nicely for you:
$ lnstat -s1 -i1 -c-1 -f rt_cache rt_cache|rt_cache|rt_cache|rt_cache|rt_cache|rt_cache|rt_cache|rt_cache|rt_cache|rt_cache|rt_cache|rt_cache|rt_cache|rt_cache|rt_cache|rt_cache|rt_cache| entries| in_hit|in_slow_|in_slow_|in_no_ro| in_brd|in_marti|in_marti| out_hit|out_slow|out_slow|gc_total|gc_ignor|gc_goal_|gc_dst_o|in_hlist|out_hlis| | | tot| mc| ute| | an_dst| an_src| | _tot| _mc| | ed| miss| verflow| _search|t_search| 3096848| 42309| 686| 0| 0| 0| 0| 0| 3| 0| 0| 674| 672| 0| 0| 27644| 8| 3096984| 41405| 636| 0| 0| 0| 0| 0| 3| 0| 0| 623| 621| 0| 0| 28189| 8| 3097160| 42483| 700| 0| 0| 0| 0| 0| 5| 0| 0| 694| 692| 0| 0| 29506| 12|
Except for the first column,
lnstat outputs values in units per
second. Let’s review some of these values:
rt_cache_entriesis the number of entries in the route cache. You should compare it with
net.ipv4.route.max_sizeand ensure that the cache is never full to avoid triggering the garbage collector too often.
rt_cache_out_hitare the number of regular lookups avoided because the result was found in the cache for incoming and outgoing packets, respectively. When the system is a router, most lookups only happen on the incoming side. You should compare this value with
rt_cache_in_out_slow_totwhich are the number of lookups in the route tables. On this system, the efficiency of the route cache is more than 98% which is quite good.
rt_cache_gc_totalis how often the garbage collector was requested to be triggered while
rt_cache_gc_ignoredcorresponds to how often it was finally not run because it has already been triggered shortly beforehand (less than
net.ipv4.route.gc_min_interval_msmilliseconds). The difference between these two values should be very small to ensure that the garbage collector does not run more a handful of times per second.
rt_cache_gc_goal_missis how often the garbage collector was not able to fulfill its goal. This value should rarely be different of 0.
rt_cache_gc_dst_overflowis how often the route cache is bigger than the allowed maximum size. This should never happen except when you try to shrink the cache.
rt_cache_out_hlist_searchis how often a lookup in the cache had to look at the next entry in the chain for the computed bucket: each time Linux has to follow the
nextpointer in a cache entry, it increments one of these counters. It is a clue on the average length of chains in the route cache hash table. Compare these values with the number of cache lookups (hit and miss).
As an illustration, here is a plot of the various statistics exposed above for a router whose route cache receives about 1000 routes par second:
rhash_entries is 1,048,576, as is
net.ipv4.route.gc_thresh. Therefore, the garbage collector was
requested to be run when the number of entries goes above this
level. Because the cache is not full, it is only really run twice per
seconds (the value of
net.ipv4.route.gc_elasticity is set to 3. This explains why
the garbage collector is aggressive when the number of entries reached
As you can see, the efficiency is near 100% all the time. The
percentage of collisions is
rt_cache_in_hlist_search ratio to the
The plot above was with a 2.6.39 kernel. For a
kernel between 2.6.35 and 2.6.37 (included) or a 3.2 kernel or more
recent, the cleanup triggered every
seconds will expire up to
rhash_entries entries. If the pace at
which routes are added to the cache is less than this rate, the number
of entries may stop climbing, even when
is not met. For example, here are the same statistics with a 2.6.35
kernel for a router with about 2,500 new entries per second but with
net.ipv4.route.gc_interval enabled; the threshold of 1,048,576 is
Do you need to modify any of these values? You have two questions to ask yourself:
- How much efficiency do you want to get from the route cache?
- How much memory do you want to dedicate to the route cache?
As a rule of thumb, two millions entries eat about 500 MiB of memory on
a 64-bit system. You should be able to compute the average memory usage
and the maximum memory usage from the values of
net.ipv4.route.gc_elasticity. For example, if the route cache hash
table has 262,144 buckets, the maximum allowed number of entries in
the cache is 4,194,304 and
net.ipv4.route.gc_elasticity is set to 8,
the memory usage will be 500 MiB on average and 1 GiB max. If this is
too much, you will need to lower some values.
Look at the previous section to compute the current efficiency of your route cache. It should be above 90%. If you are dissatisfied with that, you could increase the cache size.
If you want to double the number of entries, double
rhash_entries but keep
net.ipv4.route.gc_elasticity to 8.
For the garbage collector to be efficient, the route cache should not
be filled too fast. The garbage collector should be able to cope with
this situation but this may impact performance because it needs to
walk several times the route cache to find entries to expire. Watching
gc_goal_miss may give you a hint about this: if this value starts to
be different of 0, lower
For a kernel where
matters, things become more complicated. Because the cleanup algorithm
will expire entries at a regular interval, the average number of
entries may stay low unless the number of new entries per second is
high enough. Therefore, the average number of entries may be lower
than the theoretical value computed above. Monitoring the appropriate
metrics is the key to a good tuning. If you feel that entries are
expired too fast, you may want to double
In-depth look into the garbage collector#
These different pieces of advice may have puzzled you. We need to
understand how the garbage collector works to better cope with
them. The garbage collector is triggered when a new entry needs to be
added to the cache and the number of entries is superior to
net.ipv4.route.gc_thresh. It is implemented in
rt_garbage_collect() function. It will do
nothing if it has been called less than
net.ipv4.route.gc_interval_ms milliseconds and the cache is not full
Setting a goal#
The garbage collector will first assess the situation. It will look how
the number of entries currently in the cache compares to the product
- Above this limit, it will try to remove at least
- Otherwise, it will try to remove no more than half the difference.
If you look at the previous plot, you can easily see the difference when the garbage collector is not aggressive (above 1,048,576 entries but below 3,145,728) and when it is (above 3,145,728 entries). If we assume the garbage collector is able to meet its goal, we can easily simulate its algorithm:
The first plot shows what would happen if about 2000 routes are added
per second with
rhash_entries equal to 262,144 and
net.ipv4.route.gc_elasticity set to 8. When
net.ipv4.route.gc_threshold is met, the garbage collector has almost
no effect. However, when the number of entries reaches 2,097,152, the
garbage collector sets its goal to 262,144. From this point, the number
of entries oscillates around 2 millions entries.
On the second plot,
rhash_entries is now equal to 1,048,576 but
net.ipv4.route.gc_elasticity has been set to 2. Therefore, the
aggressive part of the garbage collectors kicks at the same threshold
than for the previous plot. However, its goal is now four times larger
and the oscillations have a larger amplitude. The fact that the
aggressive part of the garbage collector kicks less often is nullified
by the fact that it needs to remove more entries each time. Because of
the slight impact on cache efficiency, it seems better to keep
net.ipv4.route.gc_elasticity around 8, or 4 if we want to keep
shorter chains (but there seems to be no improvement to do so).
The other plots show what happens when there is a sudden surge in the number of routes added or when there is a pause.
Meeting the goal#
Now that we understand how
interact, let’s look at
net.ipv4.route.gc_timeout. Once the garbage
collector has set its goal and if it is positive, it needs to choose
which entries to remove from the cache.
It walks the hash table from the position of its last run and remove
entries until its goal is met. If the entry is not current anymore
(for example, the network interface associated to it has changed its
IP configuration), it is removed. Otherwise, the system looks at the
age of the entry and its position in the chain. If the entry is the
first in the chain, it is kept only if its age is below
net.ipv4.route.gc_timeout. If it is second, it is kept only if its
age is below half of
net.ipv4.route.gc_timeout. If it is third, the
threshold is a quarter of
net.ipv4.route.gc_timeout, and so on. The
garbage collector will favor short chains.
If after a full run of the hash table, the garbage collector was not
able to meet its goal, it will start again but will behave more
aggressively, as if
net.ipv4.route.gc_timeout is set to half of its
value. It will do as many passes as necessary until its goal is met or
until there is no way to remove any entry (or until it has spent too
much time). Once the garbage collector has switched to this more
aggressive behavior, it will keep being aggressive for a few cycles
(a bit less for each cycle).
With a very large value of
net.ipv4.route.gc_timeout, the garbage
collector will have a hard time to find entries to expire. It will
need to do several passes until it is able to expire some entries. On
the other hand, if you choose a very small value, the garbage
collector may remove entries that were just added, even if there are
older entries further in the hash table.
For a more in-depth coverage of how the route cache works, look at
chapter 33 of Understanding Linux Network
Internals. Be aware that multipath route caching has
been removed (and was never really used) and asynchronous cleanup
rt_periodic_timer) does not exist anymore.
As stated earlier,
has been reinstantiated in Linux 3.2. The cleanup is a
bit different of what is done by the garbage collector but have some
similarities. It is run every
(even when there is less than
entries). It will set a goal proportional to
net.ipv4.route.gc_interval and inversely proportional to
net.ipv4.route.gc_timeout. It cannot be greater than
net.ipv4.route.gc_timeout are equal, the goal is exactly
rhash_entries. It represents the number of entries the cleanup
procedure will look at (and not the number of entries it will try to
expire). If an entry is not valid anymore or is old enough to be
removed (with the same criteria as for the garbage collector), it will
be removed. Another important thing about this cleanup algorithm is
that it will modify the maximum allowed length of a chain to the
average length it has observed plus four times the standard deviation
with a maximum equal to
net.ipv4.route.gc_elasticity. Without this
algorithm, the maximum allowed length is 20. When inserting a new
entry, if a chain with more than
entries is selected, the kernel will try to remove an element before
inserting a new one. Then, if the chain is still longer than the
maximum allowed length (longer than what is allowed by
net.ipv4.route.gc_elasticity or too long compared to other
chains6), all cache entries for the current interface are
When tuning the route cache,
net.ipv4.route.gc_threshold are related and should not be modified
net.ipv4.route.gc_thresh should be below the product
net.ipv4.route.max_size should be above this value.
Linux exposes several interesting metrics related to the route cache. Monitoring them allows one to watch the efficiency of the route cache and may uncover some anomalies (garbage collector running too often, difficulty to remove routes from the cache, …).
The route cache subsystem still evolves and some old behaviors have been dismissed. The best documentation is, unfortunately, still the code and other documentations tend to become obsolete.
IPv6 route cache garbage collection is very similar to IPv4. The main difference is that route cache entries are stored inside the routing table radix tree, not inside a dedicated structure. Starting from Linux 4.2, only PMTU exceptions create a route cache entry.
The content of this article should be valid for Linux 2.6.38, 2.6.39, 3.0 and 3.1. There are some clues for Linux 2.6.35, 2.6.36 and 2.6.37 as well as for Linux 3.2, 3.3, 3.4 and 3.5. Starting from Linux 3.6, the whole route cache has been removed. ↩︎
Linux provides a protocol-independent destination cache subsystem (DST). This component is not a generic cache layer and only enables loose coupling with external subsystems. ↩︎
Martian addresses are addresses that cannot be used as a source address, either because they are reserved for special-use (like a multicast address) or because of the use of reverse path filtering which checks if a packet received on one interface would be answered on the same interface, as defined in RFC 3704. This feature is enabled by setting
rp_filterin Linux. ↩︎
For more details, you may want to look at
In fact, the size of the hash table is always a power of two. If specified,
rhash_entriesis rounded to the next power of two and, internally, is stored as
rt_hash_mask + 1. ↩︎
This allows the kernel to guard against collision attacks. ↩︎