recv not ready (11: Recurso temporariamente indisponível)) (nginx / 1.1.19, supervisorctl, gunicorn)

Question

Tenho site no Ubuntu 12.04.5 LTS, django (supervisorctl, gunicorn). Ele funciona 1 ano sem problemas, mas hoje o meu servidor zabbix me envia mensagens sobre alertas (a cada 3-7 minutos) sobre o tempo limite (por 1-2 segundos). Eu executo o modo de depuração para o nginx:

2015/08/26 06:24:09 [debug] 24236#0: *214786 http upstream temp fd: -1
2015/08/26 06:24:09 [debug] 24236#0: *214786 http output filter "/management/verify/[email protected]&app_version=4.1&warehouse=131&only_hash=0&pin_number="
2015/08/26 06:24:09 [debug] 24236#0: *214786 http copy filter: "/management/verify/[email protected]&app_version=4.1&warehouse=131&only_hash=0&pin_number="
2015/08/26 06:24:09 [debug] 24236#0: *214786 image filter
2015/08/26 06:24:09 [debug] 24236#0: *214786 xslt filter body
2015/08/26 06:24:09 [debug] 24236#0: *214786 http postpone filter "/management/verify/[email protected]&app_version=4.1&warehouse=131&only_hash=0&pin_number=" 00007FFF740F6500
2015/08/26 06:24:09 [debug] 24236#0: *214786 http chunk: 0
2015/08/26 06:24:09 [debug] 24236#0: *214786 write new buf t:0 f:0 000000000210B710, pos 000000000049DEDA, size: 5 file: 0, size: 0
2015/08/26 06:24:09 [debug] 24236#0: *214786 http write filter: l:1 f:0 s:5
2015/08/26 06:24:09 [debug] 24236#0: *214786 http write filter limit 0
2015/08/26 06:24:09 [debug] 24236#0: *214786 writev: 5
2015/08/26 06:24:09 [debug] 24236#0: *214786 http write filter 0000000000000000
2015/08/26 06:24:09 [debug] 24236#0: *214786 http copy filter: 0 "/management/verify/[email protected]&app_version=4.1&warehouse=131&only_hash=0&pin_number="
2015/08/26 06:24:09 [debug] 24236#0: *214786 http finalize request: 0, "/management/verify/[email protected]&app_version=4.1&warehouse=131&only_hash=0&pin_number=" a:1, c:1
2015/08/26 06:24:09 [debug] 24236#0: *214786 set http keepalive handler
2015/08/26 06:24:09 [debug] 24236#0: *214786 http close request
2015/08/26 06:24:09 [debug] 24236#0: *214786 http log handler
2015/08/26 06:24:09 [debug] 24236#0: *214786 free: 00000000021005C0
2015/08/26 06:24:09 [debug] 24236#0: *214786 free: 0000000000000000
2015/08/26 06:24:09 [debug] 24236#0: *214786 free: 0000000000000000
2015/08/26 06:24:09 [debug] 24236#0: *214786 free: 0000000000000000
2015/08/26 06:24:09 [debug] 24236#0: *214786 free: 00000000020F5160, unused: 2
2015/08/26 06:24:09 [debug] 24236#0: *214786 free: 000000000210AC50, unused: 787
2015/08/26 06:24:09 [debug] 24236#0: *214786 event timer add: 4: 600000:1440570849784
2015/08/26 06:24:09 [debug] 24236#0: *214786 free: 00000000021442A0
2015/08/26 06:24:09 [debug] 24236#0: *214786 free: 00000000020B4530
2015/08/26 06:24:09 [debug] 24236#0: *214786 hc free: 0000000000000000 0
2015/08/26 06:24:09 [debug] 24236#0: *214786 hc busy: 0000000000000000 0
2015/08/26 06:24:09 [debug] 24236#0: *214786 tcp_nodelay
2015/08/26 06:24:09 [debug] 24236#0: *214786 reusable connection: 1
2015/08/26 06:24:09 [debug] 24236#0: *214786 post event 0000000002153D48
2015/08/26 06:24:09 [debug] 24236#0: *214786 delete posted event 0000000002153D48
2015/08/26 06:24:09 [debug] 24236#0: *214786 http keepalive handler
2015/08/26 06:24:09 [debug] 24236#0: *214786 malloc: 00000000020B4530:1024
2015/08/26 06:24:09 [debug] 24236#0: *214786 recv: fd:4 -1 of 1024
2015/08/26 06:24:09 [debug] 24236#0: *214786 recv() not ready (11: Resource temporarily unavailable)
2015/08/26 06:24:09 [debug] 24236#0: *214786 post event 0000000002153D48
2015/08/26 06:24:09 [debug] 24236#0: *214786 post event 00000000021BBD58
2015/08/26 06:24:09 [debug] 24236#0: *214786 delete posted event 00000000021BBD58
2015/08/26 06:24:09 [debug] 24236#0: *214786 http empty handler
2015/08/26 06:24:09 [debug] 24236#0: *214786 delete posted event 0000000002153D48
2015/08/26 06:24:09 [debug] 24236#0: *214786 http keepalive handler
2015/08/26 06:24:09 [debug] 24236#0: *214786 recv: fd:4 0 of 1024
2015/08/26 06:24:09 [info] 24236#0: *214786 client 148.251.XXX.XXX closed keepalive connection
2015/08/26 06:24:09 [debug] 24236#0: *214786 close http connection: 4
2015/08/26 06:24:09 [debug] 24236#0: *214786 event timer del: 4: 1440570849784
2015/08/26 06:24:09 [debug] 24236#0: *214786 reusable connection: 0

O que está causando esse problema?