Reading/Web/Lazy loading of images on Ukrainian and Farsi Wikipedia

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Rollout of lazy loaded images on fa.m.wikipedia.org and uk.m.wikipedia.org suggests a nontrivial speed improvement in page fully loaded time (initial lag excluded) and a significant reduction in image bytes shipped per pageview, leading to lighter weight pages.

Note well that the x-axis in the speed graphs is presented in log2n increments.

To reduce noise, navigation timing events are filtered to ensure key fields are present, with an emphasis on anonymous, non-redirected, plain article pageviews.

To simplify analysis for bytes shipped calculations, eligible source HTML pages are constrained as to a relative path /wiki/ (on language variant wikis this path would be different, but that's out of scope here) for a given wiki without a colon ":" character in the remainder of the path, with a restriction that responses must be HTTP 200s, in order to avoid overcounting of 30x, 40x, or other such spurious responses. This aids in narrowing down the analysis to requests likely to be plain article pageviews. Image bytes are constrained to those served from upload.wikimedia.org with an eligible Referer for the same restriction as page paths.

Changes introduced to support lazy loaded images required modified (increased) JavaScript/CSS/HTML.

ukwiki

Based on back-to-back weeks just before and just after lazy loaded image implementation, pages with lazy loaded images loaded faster at the 10th, 50th (median), and 90th percentiles on both HTTP1 and HTTP2.

Percentile HTTP1 w/o lazy HTTP1 LAZY HTTP2 w/o lazy HTTP2 LAZY H1 LAZY Improvement H2 LAZY Improvement
10% 1363.6 1067.8 913 813.6 21.69% 10.89%
50% 3663 2768 2330 2176 24.43% 6.61%
90% 25140 12236.2 11497.6 7312.8 51.33% 36.40%

Based on examination of two 3-day periods (3-5 May 2016 vs 28-30 May 2016), image bytes per pageview were reduced by about 44.82%. This contributed to a decrease of about 16.05% in bytes shipped for the modified JavaScript/CSS/HTML plus images as compared to the baseline JavaScript/CSS/HTML and images.

fawiki

Based on back-to-back weeks just before and just after lazy loaded image implementation, pages with lazy loaded images loaded faster at the 10th, 50th (median), and 90th percentiles on both HTTP1 and HTTP2.

Percentile HTTP1 w/o lazy HTTP1 LAZY HTTP2 w/o lazy HTTP2 LAZY H1 LAZY Improvement H2 LAZY Improvement
10% 1294.2 1254.4 904.2 851.5 3.08% 5.83%
50% 3593 3494 2513.5 2265 2.76% 9.89%
90% 13657.2 11962.8 8360.6 7355 12.41% 14.78%

Based on examination of two 3-day periods (3-5 May 2016 vs 28-30 May 2016), image bytes per pageview were reduced by about 39.55%. This contributed to a decrease of about 16.79% in bytes shipped for the modified JavaScript/CSS/HTML plus images as compared to the baseline JavaScript/CSS/HTML and images.

Data Transfer

month day uri_host Page & scoped rl bytes image bytes scoped pageviews Page & rl per pv image per pv total avg total reduction image reduction
5 3 fa.m.wikipedia.org 25510357976 29942997370 1308953
5 4 fa.m.wikipedia.org 24115284582 28472368544 1246904
5 5 fa.m.wikipedia.org 27267566863 32394660359 1419181 19344.0187039721 22845.0712352938 42189.089939266
6 28 fa.m.wikipedia.org 31630381463 20520208225 1492149
6 29 fa.m.wikipedia.org 31082719125 20309386390 1459244
6 30 fa.m.wikipedia.org 31852934478 20497607259 1489397 21294.8675947298 13809.9756741481 35104.8432688778 0.167916555692157 0.395494304573989
5 3 uk.m.wikipedia.org 10841189176 12179570910 400598
5 4 uk.m.wikipedia.org 14113462369 16249108153 557125
5 5 uk.m.wikipedia.org 14287897826 16625942125 561363 25833.000482527 29659.0325945996 55492.0330771266
6 28 uk.m.wikipedia.org 8827165805 4991530453 293109
6 29 uk.m.wikipedia.org 8598608509 4567136094 288522
6 30 uk.m.wikipedia.org 8304006013 4373181283 269711 30222.6136229623 16364.5724397481 46587.1860627104 0.160470729231342 0.44824321604044

Queries

The following query was used to derive the lag-excluded load time, roughLoadTimeInitialLagExcluded.

select
left(timestamp,8) as ts,
event_lazyLoadImages,
event_isHttp2,
event_loadEventEnd-event_responseStart as roughLoadTimeInitialLagExcluded,
event_responseEnd-event_responseStart as roughNetworkTimeInitialLagExcluded,
event_requestStart,
event_responseStart,
event_responseEnd,
event_firstPaint,
event_domInteractive,
event_domComplete,
event_loadEventStart,
event_loadEventEnd,
webHost,
event_originCountry,
event_mediaWikiVersion
from NavigationTiming_15485142
where
timestamp > '20160412'
and timestamp < '20160630'
and event_action = 'view'
and event_isAnon = true
and event_mobileMode = 'stable'
and event_namespaceId = 0
and event_redirectCount is null
and event_loadEventEnd is not null
and event_domComplete is not null
and event_domInteractive is not null
and event_responseStart is not null
and wiki in ('bnwiki', 'fawiki', 'ukwiki')
order by
wiki
event_lazyLoadImages,
event_isHttp2,
ts
;

The following query was used to derive image bytes transferred using the constraints described above.

select
month,
day,
substr(referer,1,26),
sum(response_size)
from
webrequest
where
year = 2016
and ((month = 5 and day in (3, 4, 5)) or (month = 6 and day in (28, 29, 30)))
and uri_host = 'upload.wikimedia.org'
and referer rlike '^https://(bn|fa|uk).m.wikipedia.org/wiki/([^:])+$'
and content_type rlike '^image'
and agent_type = 'user'
and http_status = '200'
group by
month,
day,
substr(referer,1,26)
;

The following query was used to derive page and JavaScript/CSS bytes (pre- and post-modification for lazy loading) transferred using the constraints described above.

select
month,
day,
uri_host,
sum(response_size)
from
webrequest
where
year = 2016
and ((month = 5 and day in (3, 4, 5)) or (month = 6 and day in (28, 29, 30)))
and (
(
uri_host = 'bn.m.wikipedia.org'
and ((uri_path rlike '^/wiki/([^:])+$') or (uri_path = '/w/load.php' and uri_query rlike 'skins\.minerva\.icons\.images\.scripts&skin=minerva' and referer rlike '^https://bn.m.wikipedia.org/wiki/([^:])+$'))
)
or
(
uri_host = 'fa.m.wikipedia.org'
and ((uri_path rlike '^/wiki/([^:])+$') or (uri_path = '/w/load.php' and uri_query rlike 'skins\.minerva\.icons\.images\.scripts&skin=minerva' and referer rlike '^https://fa.m.wikipedia.org/wiki/([^:])+$'))
)
or
(
uri_host = 'uk.m.wikipedia.org'
and ((uri_path rlike '^/wiki/([^:])+$') or (uri_path = '/w/load.php' and uri_query rlike 'skins\.minerva\.icons\.images\.scripts&skin=minerva' and referer rlike '^https://uk.m.wikipedia.org/wiki/([^:])+$'))
)
)
and agent_type = 'user'
and http_status = '200'
group by
month,
day,
uri_host
;


The following query was used to derive pageviews for the using the constraints above. Practically all matching records were qualified as pageviews, largely ruling out the possibility of image byte transfer counts with proper Referer values being derived from anything other than qualified pageviews.

select
month,
day,
uri_host,
content_type,
is_pageview,
count(1)
from
webrequest
where
year = 2016
and ((month = 5 and day in (3, 4, 5)) or (month = 6 and day in (28, 29, 30)))
and (
(
uri_host = 'bn.m.wikipedia.org'
and uri_path rlike '^/wiki/([^:])+$'
)
or
(
uri_host = 'fa.m.wikipedia.org'
and uri_path rlike '^/wiki/([^:])+$'
)
or
(
uri_host = 'uk.m.wikipedia.org'
and uri_path rlike '^/wiki/([^:])+$'
)
)
and agent_type = 'user'
and http_status = '200'
group by
month,
day,
uri_host,
content_type,
is_pageview
;

As with any data spanning time series and the myriad complexities involved with different devices and environments, data are subject to fluctuation. However, the data transfer savings are unambiguous, and the larger event sampling pool with ukwiki and fawiki lend a degree of confidence that pages are actually loading faster.