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 performances 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.

I hope to provide here a concise view of the route cache, as it is implemented in Linux 3.11. It is protocol-dependent2 and I will cover only the IPv4 version here.

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 address3?
  • 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?

Those 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 203.0.113.15 to 198.51.100.16, 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 198.51.100.17 but maybe there is some policy routing requesting the use of a special route table or 198.51.100.16 is a local address and the packet will therefore be classified as martian.

The schema below shows how this cache is implemented4. It uses a separate hash table (BSD systems keep the cache in the routing table). Each bucket is a chained list of route cache entries.

Schema of the route cache hash table

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.

Available knobs

There are several values you can tune. Most of them are available in sysctl.

  • rhash_entries is the size of the hash table5. 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_size is the maximum number of entries in the route cache. Except under exceptional circumstances, this value is never exceeded.
  • net.ipv4.route.gc_elasticity is 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_ms is 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_thresh is a threshold triggering the garbage collector every net.ipv4.route.gc_min_interval_ms milliseconds.
  • net.ipv4.route.gc_timeout is 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 those obsolete sysctl values:

  • net.ipv4.route.secret_interval has 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_interval has 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.

UPDATED: net.ipv4.route.gc_interval is back for Linux 3.2. It is still needed to avoid exhausting the neighbour cache because it allows 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 those values:

  • rt_cache_entries is the number of entries in the route cache. You should compare it with net.ipv4.route.max_size and ensure that the cache is never full to avoid triggering the garbage collector too often.
  • rt_cache_in_hit and rt_cache_out_hit are 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_slow_tot and rt_cache_in_out_slow_tot which 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_total is how often the garbage collector was requested to be triggered while rt_cache_gc_ignored corresponds to how often it was finally not run because it has already been triggered shortly beforehand (less than net.ipv4.route.gc_min_interval_ms milliseconds). The difference between those 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_miss is how often the garbage collector was not able to fulfill its goal. This value should rarely be different of 0.
  • rt_cache_gc_dst_overflow is 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_in_hlist_search and rt_cache_out_hlist_search is 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 next pointer in a cache entry, it increments one of those counters. It is a clue on the average length of chains in the route cache hash table. Compare those 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:

Plot of various statistics for the route cache

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_interval_ms is 500). net.ipv4.route.gc_elasticity is set to 3. This explains why the garbage collector is aggressive when the number of entries reached 3,145,728.

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 sum of rt_cache_in_hit and rt_cache_in_slow_tot.

UPDATED: 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 net.ipv4.route.gc_interval 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 net.ipv4.route.gc_threshold 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 never met:

Plot of various statistics for the route cache with gc_interval

Tuning

Do you need to modify any of those values? You have two questions to ask yourself:

  1. How much efficiency do you want to get from the route cache?
  2. How much memory do you want to dedicate to the route cache?

As a rule of thumb, two millions entries eat about 500 MB of memory on a 64bit system. You should be able to compute the average memory usage and the maximum memory usage from the values of net.ipv4.route.max_size, rhash_entries and 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 MB on average and 1 GB 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 net.ipv4.route.max_size, net.ipv4.route.gc_thresh and 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 net.ipv4.route.gc_timeout.

UPDATED: For a kernel where net.ipv4.route.gc_interval 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 net.ipv4.route.gc_timeout.

In-depth look into the garbage collector

Those 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 (more than net.ipv4.route.max_size entries).

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 of rhash_entries and net.ipv4.route.gc_elasticity:

  1. Above this limit, it will try to remove at least rhash_entries entries.
  2. 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:

Plot showing influence of various kernel parameters

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 rhash_entries, net.ipv4.route.gc_elasticity and net.ipv4.route.gc_threshold 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. Just be aware that multipath route caching has been removed (and was never really used) and asynchronous cleanup (rt_periodic_timer) does not exist anymore.

UPDATED: As stated earlier, net.ipv4.route.gc_interval 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 net.ipv4.route.gc_interval seconds (even when there is less than net.ipv4.route.gc_threshold 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 rhash_entries. If net.ipv4.route.gc_interval and 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 net.ipv4.route.gc_elasticity 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 invalidated.

Conclusion

When tuning the route cache, rhash_entries, net.ipv4.route.gc_elasticity, net.ipv4.route.max_size and net.ipv4.route.gc_threshold are related and should not be modified independently. net.ipv4.route.gc_thresh should be below the product of net.ipv4.route.gc_elasticity and rhash_entries while 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. The IPv6 route cache is quite different of the one presented here. Don’t blindly apply the same tuning for IPv6.


  1. 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

  2. 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. 

  3. 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_filter in Linux. 

  4. For more details, you may want to look at include/net/dst.h,
    include/net/route.h and net/ipv4/route.c

  5. In fact, the size of the hash table is always a power of two. If specified, rhash_entries is rounded to the next power of two and, internally, is stored as rt_hash_mask + 1

  6. This allows the kernel to guard against collision attacks.