{"action":"created","comment":{"url":"https://api.github.com/repos/harvard-cs205/cs205-homework/pulls/comments/45011646","id":45011646,"diff_hunk":"@@ -0,0 +1,86 @@\n+>>Compare performance with 4 and with 1 thread (i.e., serial performance), and\n+discuss the reasons for any performance difference.\n+\n+With one thread, we get roughly 12 fps. With 4 threads, we get roughly\n+16 fps. This is a very small speedup; we naively expect that N cores\n+should speed us up N times. The overhead of parallelizing the code must be gigantic here, resulting in inefficient scaling.\n+Also, the simulation is likely incorrect as we have not implemented locks; this could result in bizarre behavior,\n+slowing down the simulation as additional threads are added. Qualitatively, the behavior of the simulation\n+still looks correct, however.\n+\n+>>Spatial Decomposition\n+\n+Spatial decomposition *dramatically* increases the FPS of our simulation. With one thread, we get\n+about 700 fps, while with four threads we find something like 800-900fps. Adding additional threads\n+does not seem to have a gigantic impact, surprisingly. It is not surprising that we see large\n+speedups here, as our algorithm changed from O(N^2) to O(N) when N=10^4.\n+\n+>>Spatially Coherent Sorting\n+\n+I use Morton order to sort the positions by utilizing an excellent module written by the Lawrence Livermore National Lab\n+(see morton.py). Reading about Morton order on Wikipedia, \"Morton order maps multidimensional data\n+to one dimension while preserving the locality of the data points.\" It basically uses a multi-dimensional fractal that\n+efficiently maps data from N dimensions to one while preserving data locality in N dimensions. Our strategy is consequently to take\n+the 2d position data and sort it based on Morton order; this way, points close in space are close in memory. This is\n+ advantageous as points close in space frequently interact with each other; we want memory lookups between them to be\n+ very fast.","path":"HW2/P5/P5.txt","position":26,"original_position":26,"commit_id":"800ddaf90e91f0b65332f9b031c4353bb7659d15","original_commit_id":"800ddaf90e91f0b65332f9b031c4353bb7659d15","user":{"login":"lhoangcse","id":14171813,"avatar_url":"https://avatars.githubusercontent.com/u/14171813?v=3","gravatar_id":"","url":"https://api.github.com/users/lhoangcse","html_url":"https://github.com/lhoangcse","followers_url":"https://api.github.com/users/lhoangcse/followers","following_url":"https://api.github.com/users/lhoangcse/following{/other_user}","gists_url":"https://api.github.com/users/lhoangcse/gists{/gist_id}","starred_url":"https://api.github.com/users/lhoangcse/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/lhoangcse/subscriptions","organizations_url":"https://api.github.com/users/lhoangcse/orgs","repos_url":"https://api.github.com/users/lhoangcse/repos","events_url":"https://api.github.com/users/lhoangcse/events{/privacy}","received_events_url":"https://api.github.com/users/lhoangcse/received_events","type":"User","site_admin":false},"body":"nice explanation","created_at":"2015-11-17T01:29:38Z","updated_at":"2015-11-17T01:29:38Z","html_url":"https://github.com/harvard-cs205/cs205-homework/pull/293#discussion_r45011646","pull_request_url":"https://api.github.com/repos/harvard-cs205/cs205-homework/pulls/293","_links":{"self":{"href":"https://api.github.com/repos/harvard-cs205/cs205-homework/pulls/comments/45011646"},"html":{"href":"https://github.com/harvard-cs205/cs205-homework/pull/293#discussion_r45011646"},"pull_request":{"href":"https://api.github.com/repos/harvard-cs205/cs205-homework/pulls/293"}}},"pull_request":{"url":"https://api.github.com/repos/harvard-cs205/cs205-homework/pulls/293","id":49471541,"html_url":"https://github.com/harvard-cs205/cs205-homework/pull/293","diff_url":"https://github.com/harvard-cs205/cs205-homework/pull/293.diff","patch_url":"https://github.com/harvard-cs205/cs205-homework/pull/293.patch","issue_url":"https://api.github.com/repos/harvard-cs205/cs205-homework/issues/293","number":293,"state":"open","locked":false,"title":"HW2 re-submission","user":{"login":"btweinstein","id":5347518,"avatar_url":"https://avatars.githubusercontent.com/u/5347518?v=3","gravatar_id":"","url":"https://api.github.com/users/btweinstein","html_url":"https://github.com/btweinstein","followers_url":"https://api.github.com/users/btweinstein/followers","following_url":"https://api.github.com/users/btweinstein/following{/other_user}","gists_url":"https://api.github.com/users/btweinstein/gists{/gist_id}","starred_url":"https://api.github.com/users/btweinstein/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/btweinstein/subscriptions","organizations_url":"https://api.github.com/users/btweinstein/orgs","repos_url":"https://api.github.com/users/btweinstein/repos","events_url":"https://api.github.com/users/btweinstein/events{/privacy}","received_events_url":"https://api.github.com/users/btweinstein/received_events","type":"User","site_admin":false},"body":"My last pull request included HW0, HW1, and HW2. This pull request just has the files for HW2 as desired. This should make the homework easier to grade!","created_at":"2015-11-02T19:06:09Z","updated_at":"2015-11-17T01:29:38Z","closed_at":null,"merged_at":null,"merge_commit_sha":"4a56bbf70d122d61d8ccbd5f68d8aae06f5a1670","assignee":null,"milestone":null,"commits_url":"https://api.github.com/repos/harvard-cs205/cs205-homework/pulls/293/commits","review_comments_url":"https://api.github.com/repos/harvard-cs205/cs205-homework/pulls/293/comments","review_comment_url":"https://api.github.com/repos/harvard-cs205/cs205-homework/pulls/comments{/number}","comments_url":"https://api.github.com/repos/harvard-cs205/cs205-homework/issues/293/comments","statuses_url":"https://api.github.com/repos/harvard-cs205/cs205-homework/statuses/800ddaf90e91f0b65332f9b031c4353bb7659d15","head":{"label":"btweinstein:HW2_fixed","ref":"HW2_fixed","sha":"800ddaf90e91f0b65332f9b031c4353bb7659d15","user":{"login":"btweinstein","id":5347518,"avatar_url":"https://avatars.githubusercontent.com/u/5347518?v=3","gravatar_id":"","url":"https://api.github.com/users/btweinstein","html_url":"https://github.com/btweinstein","followers_url":"https://api.github.com/users/btweinstein/followers","following_url":"https://api.github.com/users/btweinstein/following{/other_user}","gists_url":"https://api.github.com/users/btweinstein/gists{/gist_id}","starred_url":"https://api.github.com/users/btweinstein/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/btweinstein/subscriptions","organizations_url":"https://api.github.com/users/btweinstein/orgs","repos_url":"https://api.github.com/users/btweinstein/repos","events_url":"https://api.github.com/users/btweinstein/events{/privacy}","received_events_url":"https://api.github.com/users/btweinstein/received_events","type":"User","site_admin":false},"repo":{"id":41835898,"name":"cs205-homework","full_name":"btweinstein/cs205-homework","owner":{"login":"btweinstein","id":5347518,"avatar_url":"https://avatars.githubusercontent.com/u/5347518?v=3","gravatar_id":"","url":"https://api.github.com/users/btweinstein","html_url":"https://github.com/btweinstein","followers_url":"https://api.github.com/users/btweinstein/followers","following_url":"https://api.github.com/users/btweinstein/following{/other_user}","gists_url":"https://api.github.com/users/btweinstein/gists{/gist_id}","starred_url":"https://api.github.com/users/btweinstein/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/btweinstein/subscriptions","organizations_url":"https://api.github.com/users/btweinstein/orgs","repos_url":"https://api.github.com/users/btweinstein/repos","events_url":"https://api.github.com/users/btweinstein/events{/privacy}","received_events_url":"https://api.github.com/users/btweinstein/received_events","type":"User","site_admin":false},"private":false,"html_url":"https://github.com/btweinstein/cs205-homework","description":"","fork":true,"url":"https://api.github.com/repos/btweinstein/cs205-homework","forks_url":"https://api.github.com/repos/btweinstein/cs205-homework/forks","keys_url":"https://api.github.com/repos/btweinstein/cs205-homework/keys{/key_id}","collaborators_url":"https://api.github.com/repos/btweinstein/cs205-homework/collaborators{/collaborator}","teams_url":"https://api.github.com/repos/btweinstein/cs205-homework/teams","hooks_url":"https://api.github.com/repos/btweinstein/cs205-homework/hooks","issue_events_url":"https://api.github.com/repos/btweinstein/cs205-homework/issues/events{/number}","events_url":"https://api.github.com/repos/btweinstein/cs205-homework/events","assignees_url":"https://api.github.com/repos/btweinstein/cs205-homework/assignees{/user}","branches_url":"https://api.github.com/repos/btweinstein/cs205-homework/branches{/branch}","tags_url":"https://api.github.com/repos/btweinstein/cs205-homework/tags","blobs_url":"https://api.github.com/repos/btweinstein/cs205-homework/git/blobs{/sha}","git_tags_url":"https://api.github.com/repos/btweinstein/cs205-homework/git/tags{/sha}","git_refs_url":"https://api.github.com/repos/btweinstein/cs205-homework/git/refs{/sha}","trees_url":"https://api.github.com/repos/btweinstein/cs205-homework/git/trees{/sha}","statuses_url":"https://api.github.com/repos/btweinstein/cs205-homework/statuses/{sha}","languages_url":"https://api.github.com/repos/btweinstein/cs205-homework/languages","stargazers_url":"https://api.github.com/repos/btweinstein/cs205-homework/stargazers","contributors_url":"https://api.github.com/repos/btweinstein/cs205-homework/contributors","subscribers_url":"https://api.github.com/repos/btweinstein/cs205-homework/subscribers","subscription_url":"https://api.github.com/repos/btweinstein/cs205-homework/subscription","commits_url":"https://api.github.com/repos/btweinstein/cs205-homework/commits{/sha}","git_commits_url":"https://api.github.com/repos/btweinstein/cs205-homework/git/commits{/sha}","comments_url":"https://api.github.com/repos/btweinstein/cs205-homework/comments{/number}","issue_comment_url":"https://api.github.com/repos/btweinstein/cs205-homework/issu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