Neo4j query performance with memory tuning - performance

Question

I have a graph (User-[Likes]->Item) with millions nodes and billions nodes (roughly 50G in disk) built on a powerful machine with 256G RAM and 40 cores. Currently, I'm computing the allshortestpath() between two items.
To improve the cypher query performance, I set dbms.pagecache.memory=100g and wrapper.java.additional=-Xmx32g, with the hope that the whole neo4j can be loaded into meomory. However, when I execute the shortestpath query, the CPU usage is 1625% while MEMORY usage is only 5.7%, and I didn't see performance improvements on the cypher query. Am I missing something in the setting? Or can I setup something to run the query faster? I have read the Performance Tuning guide in the developer manual but didn't find solution.
EDIT1:
The cypher query is to count the number of unique users that like both two items. The full pattern would be (Brand)-[:Has]->(Item)<-[:LIKES]-(User)-[:LIKES]->(Item)<-[:HAS]-(Brand)
profile
MATCH p = allShortestPaths((p1:Brand {FID:'001'})-[*..4]-(p2:Brand {FID:'002'}))
with [r in RELS(p)|type(r)] as relationshipPath,
[n in nodes(p)|id(n)][2] as user, p1, p2
return p1.FID, p2.FID, count(distinct user);
EDIT2:
Below is a sampler query plan. It now seems that I'm not using shortestsPath efficiently (380,556,69 db hits). I use shortestsPath to get the common user node between start/end nodes, and then use count(distinct) to get the unique user. Is it possible to tell cypher to eliminate paths which contain the node that have been visited before?

Answer 1

Can you try to run this instead:
MATCH (p1:Brand {FID:'001'}),(p2:Brand {FID:'002'})
MATCH (u:User)
WHERE (p1)-[:Has]->()<-[:LIKES]-(u) AND
(p2)-[:Has]->()<-[:LIKES]-(u)
RETURN p1,p2,count(u);
This starts at the user and checks against both brands, the explain plan looks much better
+----------------------+----------------+------------------------------------------+---------------------------+
| Operator | Estimated Rows | Variables | Other |
+----------------------+----------------+------------------------------------------+---------------------------+
| +ProduceResults | 0 | count(u), p1, p2 | p1, p2, count(u) |
| | +----------------+------------------------------------------+---------------------------+
| +EagerAggregation | 0 | count(u) -- p1, p2 | p1, p2 |
| | +----------------+------------------------------------------+---------------------------+
| +SemiApply | 0 | p2 -- p1, u | |
| |\ +----------------+------------------------------------------+---------------------------+
| | +Expand(Into) | 0 | anon[78] -- anon[87], anon[89], p1, u | (p1)-[:Has]->() |
| | | +----------------+------------------------------------------+---------------------------+
| | +Expand(All) | 0 | anon[87], anon[89] -- p1, u | (u)-[:LIKES]->() |
| | | +----------------+------------------------------------------+---------------------------+
| | +Argument | 1 | p1, u | |
| | +----------------+------------------------------------------+---------------------------+
| +SemiApply | 0 | p1 -- p2, u | |
| |\ +----------------+------------------------------------------+---------------------------+
| | +Expand(Into) | 0 | anon[119] -- anon[128], anon[130], p2, u | (p2)-[:Has]->() |
| | | +----------------+------------------------------------------+---------------------------+
| | +Expand(All) | 0 | anon[128], anon[130] -- p2, u | (u)-[:LIKES]->() |
| | | +----------------+------------------------------------------+---------------------------+
| | +Argument | 1 | p2, u | |
| | +----------------+------------------------------------------+---------------------------+
| +CartesianProduct | 0 | u -- p1, p2 | |
| |\ +----------------+------------------------------------------+---------------------------+
| | +CartesianProduct | 0 | p2 -- p1 | |
| | |\ +----------------+------------------------------------------+---------------------------+
| | | +Filter | 0 | p1 | p1.FID == { AUTOSTRING0} |
| | | | +----------------+------------------------------------------+---------------------------+
| | | +NodeByLabelScan | 0 | p1 | :Brand |
| | | +----------------+------------------------------------------+---------------------------+
| | +Filter | 0 | p2 | p2.FID == { AUTOSTRING1} |
| | | +----------------+------------------------------------------+---------------------------+
| | +NodeByLabelScan | 0 | p2 | :Brand |
| | +----------------+------------------------------------------+---------------------------+
| +NodeByLabelScan | 0 | u | :User |
+----------------------+----------------+------------------------------------------+---------------------------+