Thanks for the detailed report, Faidon.

Can you clarify something: for a given set of heavyweight thumbnails that need to be rendered, assuming the the uploads have ceased, would multiple visits of Special:NewFiles in a short timeframe multiply the saturation by the amount of HTTP requests to the same thumbnail URLs? I.e. if you request the URL of a thumbnail which is currently being generated because someone else requested it, does it make the issue worse?

Second question is, how come piling on jobs doesn't just make the jobs that came last complete much later? The same kind of DoS situation could happen with someone bombarding us with HEAD requests on previously unrequested thumbnail sizes for small images, so I think that the issue isn't specific to large jobs. It's more a matter of properly queueing things up so that the imagescalers don't overload, regardless of the mix of job weight.

On Mon, Apr 21, 2014 at 12:05 PM, Faidon Liambotis <faidon@wikimedia.org> wrote:

On Mon, Apr 21, 2014 at 10:56:40AM +0200, Giuseppe Lavagetto wrote:
> The problem resolved before I could get to strace the apache processes, so
> I don't have more details - Faidon was investigating as well and may have
> more info.

Indeed, I do: this had nothing to do with TMH. The trigger was Commons
User:Fć uploading hundreds of 100-200MB multipage TIFFs via GWToolset
over the course of 4-5 hours (multiple files per minute), and then
random users/bots viewing Special:NewFiles, which attempts to display a
thumbnail for all of those new files in parallel in realtime, and thus
saturating imagescalers' MaxClients setting and basically inadvertently
DoSing them.

The issue was temporary because of
https://bugzilla.wikimedia.org/show_bug.cgi?id=49118 but since the user
kept uploading new files, it was recurrent, with different files every
time. Essentially, we would keep having short outages every now and then
for as long as the upload activity continued.

I left a comment over at https://commons.wikimedia.org/wiki/User_talk:Fć
and contacted Commons admins over at #wikimedia-commons, as a courtesy
to both before I used my root to elevate my privileges and ban a
long-time prominent Wikimedia user as an emergency countermeasure :)

It was effective, as Fć immediately responded and ceased the activity
until further discussion; the Commons community was also helpful in the
short discussion that followed.

Andre also pointed out that Fć had previously began the "Images so big
they break Commons" thread at the Commons Village Pump:
https://commons.wikimedia.org/wiki/Commons:Village_pump#Images_so_big_they_break_Commons.3F

As for the more permanent solution: there's not much we, as ops, can do
about this but say "no, don't upload all these files", which is
obviously not a great solution :) The root cause is an architecture
issue with how imagescalers behave with regards to resource-intensive
jobs coming in a short period of time. Perhaps a combination of
poolcounter per file and more capacity (servers) would alleviate the
effect, but ideally we should be able to have some grouping &
prioritization of imagescaling jobs so that large jobs can't completely
saturate and DoS the cluster.

Aaron/Multimedia team, what do you think?

Regards,
Faidon

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