Logstash Input UDP problem

First of all sorry for my bad english.

I'm doing logstash input UDP performance test.
Unfortunately every time when i proceed logstash, i'm facing data loss. not sure where i am going wrong.

I Followed below steps.
1.Send packets to the server(515 port)
2.logstash input udp handle the packets and output generate file.

My logstash configuration

input {
udp {
workers => 8
buffer_size => 81920
queue_size => 15000
port => 515
type => "performance"

filter {}

output {
file {
#flush_interval => 0
path => ["/logstash.log"]

tcpdump result

logstash output result

tcpdump shows 10000 packets captured but logstash output file show only 6955 lines.
Logstash is running on 8 core machine.

Can anyone suggest me where the configuration is wrong?

UDP is not guaranteed delivery.