failed (104: Connection reset by peer) while reading response header from upstream, client:
If you are getting the above error in nginx logs running in from of upstream servers you may consider doing this as it worked for me:
check the ulimit on the machines and ensure it is high enough to handle the load coming in. 'ulimit' on linux, I am told determines the maximum number of open files the kernel can handle.

The way I did that?
modifying limits: for open files:
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add or change this line in /etc/systcl.conf
fs.file-max = <limit-number>

set soft & hard limits in this file for various users: /etc/security/limits.conf
<user-name or group-name or *> soft nofile <limit-number>
<user-name or group-name or *> soft nofile <limit-number> hard nofile <limit-number>

Then reload the system settings withou restarting:
sysctl -p


nginx settings :
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in nginx.conf add/set:
worker_rlimit_nofile <limit-number>

When setting up upstream servers or not, it is a good idea to keep some connections alive when running on HTTP 1.1. Consider adding this to the upstream block on :
keepalive <some-number>;