Necko/MobileCache/MicroBenchmarks
This Is Work In Progress - Information Is Incomplete
This page describes the cache-related microbenchmarks created so far. Contact BjarneG (bherland@mozilla.com) for a patch which lets you run these benchmark using the "check-one" target of xpcshell tests.
Each benchmark below is described and explained, and we also show output results from selected platforms. Note that results are provided as examples and information only; the benchmarks are meant for evaluating effect of code-changes, i.e. you run the benchmarks without and with your change and compare the results to evaluate the effect of your code.
test_timing_cache.js
This benchmark measures time to load a resource from a server and from the cache, as well as time for call-overhead and clearing the cache.
The approach is to repeat a single load in a loop for some time and report the average time for one iteration. This eliminates timer-granularity issues and should smoothen out random jitter in the system, but the weakness is that IO-caching at any level will influence disk-cache results. Moreover, memory-cache results are likely to be influenced by HW caches and memory-paging in the OS. In short, this benchmark does not implement a very realistic access-pattern for cache-entries, hence its practical value may be limited. It can be useful, though, to study changes unrelated to the speed of the cache media, e.g. code to optimize datastructures or algorithms.
Note that there are two different tests for cache-misses: One which clears the cache in each iteration, and one where the response has a header which prevents it from being cached. (The first is mainly present for completeness - it makes a clearCache() call in each iteration, which is not very likely to happen in real life.) One could also imagine a third approach to emulate cache-misses; suffix the url with a different query-string in order to load a new resource in each iteration. However, this doesn't work because it hits the max number of open connections and times out. (See test below though, where we can control this limit in a different manner.)
All operations are reported for different data-sizes, as well as with no cache enabled, memory-cache enabled only and disk-cache enabled only. (Note that, theoretically, searching for an entry in a cache should take some time, hence we measure also with no cache enabled.)
Some results from a Linux server running 64-bit Ubuntu 10.4
Memory-cache capacity: 51200 Kb / Disk-cache capacity: 1048576 Kb
Max #connections=256 / max per server=256
Total time #runs avg time/run
=== Set datasize 50 bytes
Calloverhead (no-cache) 10016 14594 0.69
Calloverhead+ClearCache (no-cache) 10015 11647 0.86
CacheMiss (nocache) (no cache) 10047 368 27.30
Calloverhead (mem-cache) 10015 14532 0.69
Calloverhead+ClearCache (mem-cache) 10016 11616 0.86
CacheMiss (clrcache) (mem-cache) 10018 360 27.83
CacheMiss (nocache) (mem-cache) 10032 367 27.34
CacheHit (mem-cache) 10017 4674 2.14
Calloverhead (disk-cache) 10015 13954 0.72
Calloverhead+ClearCache (disk-cache) 10017 6723 1.49
CacheMiss (clrcache) (disk-cache) 10033 346 29.00
CacheMiss (nocache) (disk-cache) 10032 357 28.10
CacheHit (disk-cache) 10017 4709 2.13
=== Set datasize 1024 bytes
CacheMiss (nocache) (no cache) 10020 339 29.56
CacheMiss (nocache) (mem-cache) 10037 351 28.60
CacheHit (mem-cache) 10019 3634 2.76
CacheMiss (nocache) (disk-cache) 10048 345 29.12
CacheHit (disk-cache) 10018 3535 2.83
=== Set datasize 51200 bytes
CacheMiss (nocache) (no cache) 10095 103 98.01
CacheMiss (nocache) (mem-cache) 10023 104 96.38
CacheHit (mem-cache) 10070 306 32.91
CacheMiss (nocache) (disk-cache) 10110 104 97.21
CacheHit (disk-cache) 10054 301 33.40
=== Set datasize 524288 bytes
CacheMiss (nocache) (no cache) 10277 14 734.07
CacheMiss (nocache) (mem-cache) 10312 14 736.57
CacheHit (mem-cache) 10443 31 336.87
CacheMiss (nocache) (disk-cache) 10283 14 734.50
CacheHit (disk-cache) 10451 31 337.13
The important numbers are found in the rightmost column. Note that the first block (datasize 50 bytes) includes more results than the other blocks. "Pure Overhead" measures the call-overhead from JavaScript, and "Overhead+ClearCache" measures the time to Clear the cache from JavaScript. These numbers are unrelated to the size of the data loaded, hence reported only once.
Results above indicate that reading from the disk-cache is about as fast as reading from memory-cache, which probably is due to efficient IO-caching on this platform. Note also that cache-hits for small entries are relatively much faster than cache-hits for larger entries (i.e. for small entries there is a factor 10 whereas for large entries we see a factor 2-3).
There is no conclusive evidence that searching a disk or memory cache takes any significant amount of time. However, the caches in this test are pretty much empty so we should create a separate test for this, making sure the cache contains some number of entries before we measure. (See #Wanted_tests below)
Some raw results from my Nexus S. These look a little weird...
Memory-cache capacity: 51200 Kb / Disk-cache capacity: 1048576 Kb
Max #connections=256 / max per server=256
Total time #runs avg time/run
=== Set datasize 50 bytes
Calloverhead (no-cache) 10016 10878 0.92
Calloverhead+ClearCache (no-cache) 10016 9482 1.06
CacheMiss (nocache) (no cache) 10020 321 31.21
Calloverhead (mem-cache) 10015 10975 0.91
Calloverhead+ClearCache (mem-cache) 10016 8937 1.12
CacheMiss (clrcache) (mem-cache) 10036 371 27.05
CacheMiss (nocache) (mem-cache) 10021 348 28.80
CacheHit (mem-cache) 10016 4639 2.16
Calloverhead (disk-cache) 10016 11426 0.88
Calloverhead+ClearCache (disk-cache) 10043 213 47.15
CacheMiss (clrcache) (disk-cache) 10034 117 85.76
CacheMiss (nocache) (disk-cache) 10019 347 28.87
CacheHit (disk-cache) 10017 4206 2.38
=== Set datasize 1024 bytes
CacheMiss (nocache) (no cache) 10049 332 30.27
CacheMiss (nocache) (mem-cache) 10024 339 29.57
CacheHit (mem-cache) 10017 3501 2.86
CacheMiss (nocache) (disk-cache) 10031 327 30.68
CacheHit (disk-cache) 10017 3308 3.03
=== Set datasize 51200 bytes
CacheMiss (nocache) (no cache) 10064 119 84.57
CacheMiss (nocache) (mem-cache) 10074 119 84.66
CacheHit (mem-cache) 10049 396 25.38
CacheMiss (nocache) (disk-cache) 10078 116 86.88
CacheHit (disk-cache) 10031 426 23.55
=== Set datasize 524288 bytes
CacheMiss (nocache) (no cache) 10572 19 556.42
CacheMiss (nocache) (mem-cache) 10052 18 558.44
CacheHit (mem-cache) 10274 45 228.31
CacheMiss (nocache) (disk-cache) 10167 18 564.83
CacheHit (disk-cache) 10177 43 236.67
test_timing_cache_2.js
This benchmark loads a number of resources with different cache-keys and then waits for all cache-io to finish. Then it loads all resources again (retrieving them from cache this time). Both sequences are measured and reported for various entry-sizes and cache-configurations.
This access-pattern is slightly more realistic than in the benchmark described above; a page is likely to load some number of resources from the (same) server, and if the user visits the page second time, those resources are loaded again (from cache this time). The benchmark aims at emulating this pattern in a simple way.
Some results from a Linux server running 64-bit Ubuntu 10.4
Memory-cache capacity: 51200 Kb / Disk-cache capacity: 1048576 Kb
Max #connections=256 / max per server=256
Total time #runs avg time/run
Setting datasize 50 bytes
CacheMiss (nocache) (no cache) 2803 101 27.75
CacheMiss (q-url) (mem-cache) 2767 101 27.40
CacheHit (mem-cache) 205 101 2.03
CacheMiss (q-url) (disk-cache) 2783 101 27.55
CacheHit (disk-cache) 186 101 1.84
Setting datasize 1024 bytes
CacheMiss (nocache) (no cache) 2884 101 28.55
CacheMiss (q-url) (mem-cache) 2570 101 25.45
CacheHit (mem-cache) 269 101 2.66
CacheMiss (q-url) (disk-cache) 2910 101 28.81
CacheHit (disk-cache) 271 101 2.68
Setting datasize 51200 bytes
CacheMiss (nocache) (no cache) 9546 101 94.51
CacheMiss (q-url) (mem-cache) 9536 101 94.42
CacheHit (mem-cache) 3274 101 32.42
CacheMiss (q-url) (disk-cache) 9543 101 94.49
CacheHit (disk-cache) 3274 101 32.42
Setting datasize 524288 bytes
CacheMiss (nocache) (no cache) 75942 101 751.90
CacheMiss (q-url) (mem-cache) 75840 101 750.89
CacheHit (mem-cache) 27799 101 275.24
CacheMiss (q-url) (disk-cache) 79318 101 785.33
CacheHit (disk-cache) 31333 101 310.23
The interesting numbers are found in the rightmost column of the output.
Results above suggest that loading entries up to a certain size from the server (the xpcshell-handler in this case) is rather consistent regardless of whether we use no cache, mem-cache or a disk-cache. Also, when loading entries up to a certain size from cache, the disk-cache seems to be as fast as the memory-cache. This IMO indicates that IO on this platform is pretty efficient (up to a certain limit) because we would always expect to spend more time when using the disk-cache than when using mem-cache. Observe that we need 512K-sized resources to see a clear difference between using disk and memory. (The exact limit here is not identified, but it may not very interesting either since it is probably platform-specific).
Also observe that when reading from the cache, small entries load relatively much faster than larger entries. (Memory-page size?)
Wanted tests
Unordered and informal list of tests we may also want to create
- measure time to search a cache
- mem- and disk-only, but also the combination of them I believe. Make sure we search the cache without finding the entry.
- identify entry-size where disk-cache is clearly slower than mem-cache
- probably OS- or device-specific but it may be worth finding in case we can learn something general from it
- driver which can take a list of resources and measure time to load them all
- we can extract this info from e.g. Talos or by using telemetry, and it should probably be hierarchical (some resources causes other to load)
Results from my Nexus S. Like above, these seem weird...
Memory-cache capacity: 51200 Kb / Disk-cache capacity: 1048576 Kb
Max #connections=256 / max per server=256
Total time #runs avg time/run
Setting datasize 50 bytes
CacheMiss (nocache) (no cache) 2724 101 26.97
CacheMiss (q-url) (mem-cache) 2249 101 22.27
CacheHit (mem-cache) 126 101 1.25
CacheMiss (q-url) (disk-cache) 2314 101 22.91
CacheHit (disk-cache) 146 101 1.45
Setting datasize 1024 bytes
CacheMiss (nocache) (no cache) 2190 101 21.68
CacheMiss (q-url) (mem-cache) 2206 101 21.84
CacheHit (mem-cache) 177 101 1.75
CacheMiss (q-url) (disk-cache) 2382 101 23.58
CacheHit (disk-cache) 189 101 1.87
Setting datasize 51200 bytes
CacheMiss (nocache) (no cache) 7388 101 73.15
CacheMiss (q-url) (mem-cache) 7582 101 75.07
CacheHit (mem-cache) 2205 101 21.83
CacheMiss (q-url) (disk-cache) 7749 101 76.72
CacheHit (disk-cache) 2226 101 22.04
Setting datasize 524288 bytes
CacheMiss (nocache) (no cache) 57251 101 566.84
CacheMiss (q-url) (mem-cache) 57939 101 573.65
CacheHit (mem-cache) 22888 101 226.61
CacheMiss (q-url) (disk-cache) 62850 101 622.28
CacheHit (disk-cache) 22626 101 224.02