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Here's a dump of the stats provided my mod_pagespeed from one of my sites.

resource_url_domain_rejections: 6105
rewrite_cached_output_missed_deadline: 4801
rewrite_cached_output_hits: 116004
rewrite_cached_output_misses: 934
resource_404_count: 0
slurp_404_count: 0
total_page_load_ms: 0
page_load_count: 0
resource_fetches_cached: 0
resource_fetch_construct_successes: 45
resource_fetch_construct_failures: 0
num_flushes: 947
total_fetch_count: 0
total_rewrite_count: 0
cache_time_us: 572878
cache_hits: 872
cache_misses: 1345
cache_expirations: 242
cache_inserts: 1795
cache_extensions: 50799
not_cacheable: 0
css_file_count_reduction: 0
css_elements: 0
domain_rewrites: 0
google_analytics_page_load_count: 0
google_analytics_rewritten_count: 0
image_inline: 7567
image_rewrite_saved_bytes: 208854
image_rewrites: 34128
image_ongoing_rewrites: 0
image_webp_rewrites: 0
image_rewrites_dropped_due_to_load: 0
image_file_count_reduction: 0
javascript_blocks_minified: 12438
javascript_bytes_saved: 1173778
javascript_minification_failures: 0
javascript_total_blocks: 12439
js_file_count_reduction: 0
converted_meta_tags: 902
url_trims: 54765
url_trim_saved_bytes: 1651244
css_filter_files_minified: 0
css_filter_minified_bytes_saved: 0
css_filter_parse_failures: 2
css_image_rewrites: 0
css_image_cache_extends: 0
css_image_no_rewrite: 0
css_imports_to_links: 0
serf_fetch_request_count: 1412
serf_fetch_bytes_count: 12809245
serf_fetch_time_duration_ms: 28706
serf_fetch_cancel_count: 0
serf_fetch_active_count: 0
serf_fetch_timeout_count: 0
serf_fetch_failure_count: 0

Can someone please explain what all of the stats mean?

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1 Answer 1

up vote 7 down vote accepted

There's a lot of stats here. I'm going to just describe a few of them, because this will get long. We should probably add detailed doc. I can follow-up with more answers later if these are useful.

  • resource_url_domain_rejections: 6105: this means that since your server restarted, mod_pagespeed has found 6105 resources that it's not going rewrite a resource because their domains are not authorized for rewriting with the ModPagespeedDomain directive. This is common & occurs anytime time someone refreshes a page with a twitter, facebook, or google+ widget.
  • rewrite_cached_output_missed_deadline: 4801: when a resources (e.g. a jpeg image) is optimized, it happens in a background thread, and the result is cached so that subsequent page views referencing the same refresh are fast. To avoid slowing down the first view, however, we use a 10 millisecond timer to avoid slowing down the time-to-first byte. This stat counts how many times that deadline was exceeded, in which case the resource is left unchanged for that view, but the optimization continues in the background & so the cache is written.
  • rewrite_cached_output_hits: 116004: counts the number of times we served an optimized resource from the cache, thus avoiding the need to re-optimize it.
  • rewrite_cached_output_misses: 934: counts the number of times we looked up a resource in our cache and it wasn't there, forcing us to rewrite it. Note that we would also rewrite a resource that was in the cache, but whose origin cache expiration-time had expired. E.g. if your images had cache-control:max-age=600 then we would re-fetch them every 10 minutes to see if they've changed. If they have changed we must re-optimize them.
  • num_flushes: 947: this is the number of times the Apache resource-generator for the HTML (e.g. mod_php or Wordpress) called the Apache function ap_flush(), which causes partial HTML to be flushed all the way through to the user's browser. This is interesting for mod_pagespeed because it can limit the amount of optimization we can do (e.g. we can't combine CSS files whose elements are separated by a Flush).
  • cache_time_us: 572878 - the total amount of time, in microseconds, spent waiting for mod_pagespeed's HTTP Cache (file + memory) to respond to a lookup request, since the server was started.

I think that's enough for now. Are there specific other statistics you'd like to learn more about?

Most of these were created for us to monitor the health of mod_pagespeed as it's running, and to help diagnose users' issues. I have to admit we haven't used it much for that purpose, but we use them during development.

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Awesome answer, just what I was looking for. Do you know of any other resources I could read to learn more? –  Sam Feb 3 '12 at 9:45
Great; we haven't documented the detailed statistics; this is the first I've written about it. The only other resource is the source. You might have some luck grepping the source for the statistic you have questions about, but don't hesitate to follow up with questions. mod-pagespeed-discuss is something we monitor all the time, but I'm learning to check back here on stackoverflow occasionally. –  Joshua Marantz Feb 4 '12 at 22:52
Did there end up being any more doco written for these stats? I was interested in the cache stats: cache_hits, cache_misses, cache_fallbacks, cache_expirations, file_cache_hits, file_cache_misses –  Glenn Slaven Jun 19 '13 at 2:19

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