Ok so, after a lot of battles with HHVM 3.6.1, the mysterious 503s on the imagescalers continued.

One possible cause is the fact that apparently some images require more memory to be converted on trusty:
```
curl -H 'Host: commons.wikimedia.org' 'http://mw1152/w/thumb_handler.php/1/17/HawkridgeBarton_Chittlehampton_NorthDevon.PNG/1000px-HawkridgeBarton_Chittlehampton_NorthDevon.PNG' -I
HTTP/1.1 500 Internal server error
Date: Wed, 06 May 2015 15:13:17 GMT
Server: Apache
X-Powered-By: HHVM/3.6.1
X-Content-Type-Options: nosniff
Cache-control: no-cache
X-MW-Thumbnail-Renderer: mw1152
Connection: close
Content-Type: text/html; charset=utf-8
```

which results in the kernel recording

```
May  6 15:13:20 mw1152 kernel: [4775538.759268] memory: usage 307200kB, limit 307200kB, failcnt 50
May  6 15:13:20 mw1152 kernel: [4775538.759269] memory+swap: usage 0kB, limit 18014398509481983kB, failcnt 0
May  6 15:13:20 mw1152 kernel: [4775538.759270] kmem: usage 0kB, limit 18014398509481983kB, failcnt 0
May  6 15:13:20 mw1152 kernel: [4775538.759271] Memory cgroup stats for /mediawiki/job/15592: cache:0KB rss:307200KB rss_huge:202752KB mapped_file:0KB writeback:0KB inactive_anon:0KB active_anon:307132KB inactive_file:0KB active_file:0KB unevictable:0KB
May  6 15:13:20 mw1152 kernel: [4775538.759282] [ pid ]   uid  tgid total_vm      rss nr_ptes swapents oom_score_adj name
May  6 15:13:20 mw1152 kernel: [4775538.759334] [15592]    33 15592     5222      426      16        0             0 bash
May  6 15:13:20 mw1152 kernel: [4775538.759336] [15594]    33 15594     2867      167      11        0             0 timeout
May  6 15:13:20 mw1152 kernel: [4775538.759337] [15595]    33 15595    93743    77414     186        0             0 convert
May  6 15:13:20 mw1152 kernel: [4775538.759339] Memory cgroup out of memory: Kill process 15595 (convert) score 1010 or sacrifice child
May  6 15:13:20 mw1152 kernel: [4775538.768545] Killed process 15595 (convert) total-vm:374972kB, anon-rss:307004kB, file-rss:2652kB
May  6 15:15:01 mw1152 CRON[15698]: (root) CMD (command -v debian-sa1 > /dev/null && debian-sa1 1 1)
```

Also, I tested that at smaller resulting sizes this doesn't cause the 500 to be emitted.

This is the mediawiki error log: P612

I'm still unsure this is really the cause of the larger number of 503s I see.