What do the mod_pagespeed statistics mean?

Question

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?

Answer 1

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.