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In relay.c, function cell_queues_check_size, the OOM manager attempts to clear one tenth of MaxMemInQueues bytes from the DNS cache by calling dns_cache_handle_oom. The function dns_cache_handle_oom, in dns.c, runs in a loop removing cached entries that are now+n*time_inc old, until at least the requested number of bytes have been freed. The first iteration of the loop has n=0, and likely will not remove enough bytes. The second iteration is way too aggressive, because:

time_inc += 3600; /* Increase time_inc by 1 hour. */

This is guaranteed to wipe the entire DNS cache, because in dns_clip_ttl the maximum time to cache is MAX_DNS_TTL_AT_EXIT, which is set in dns.h to:

/** Lowest value for DNS ttl that a server will give. */
#define MIN_DNS_TTL_AT_EXIT (5*60)
/** Highest value for DNS ttl that a server will give. */
#define MAX_DNS_TTL_AT_EXIT (60*60)

One possible and reasonable fix would be to instead increment time_inc by MIN_DNS_TTL_AT_EXIT.