Lemmy is incredibly unique in it's stance of not using Redis, Memcached, dragonfly... something. And all the CPU cores and RAM for what this week is reported as 57K active users across over 1200 Instance servers.

Why no Redis, Memcached, dragonfly? These are staples of API for scaling.

Anyway, Reddit too started with PostgreSQL and was open source.

MONDAY, MAY 17, 2010

http://highscalability.com/blog/2010/5/17/7-lessons-learned-while-building-reddit-to-270-million-page.html

"and growing Reddit to 7.5 million users per month"

Lesson 5: Memcache
The essence of this lesson is: memcache everything.

They store everything in memcache: 1. Database data 2. Session data 3. Rendered pages 4. Memoizing (remember previously calculated results) internal functions 5. Rate-limiting user actions, crawlers 6. Storing pre-computing listings/pages 7. Global locking.

They store more data now in Memcachedb than Postgres. It’s like memcache but stores to disk. Very fast. All queries are generated by same piece of control and is cached in memcached. Change password Links and associated state are cached for 20 minutes or so. Same for Captchas. Used for links they don’t want to store forever.

They built memoization into their framework. Results that are calculated are also cached: normalized pages, listings, everything.

1. post primary key has gaps in it, the sequence is being used for transactions that are later canceled or some kind of orphan posts? This is observable from incoming federation posts from other instances.

2. comment_aggregates has a unique id column from comment table id column. There is a one to one row relationship. Can the logic be reworked to eliminate the extra column and related INDEX overhead?

3. Related to 2... Same issue probably exist in post_aggregates table and others that have one to one join relationships.

This is the first post or comment in Lemmy history to say log_min_duration_statement ... ;)

It is possible to instruct PostgreSQL to log any query that takes over a certain amount of time, 2.5 seconds what I think would be a useful starting point. Minimizing the amount of logging activity to only those causing the most serious issues.

"Possibly the most generally useful log setting for troubleshooting performance, especially on a production server. Records only long-running queries for analysis; since these are often your "problem" queries, these are the most useful ones to know about. Used for pg_fouine." - https://postgresqlco.nf/doc/en/param/log_min_duration_statement/

I think it would really help if we could get lemmy.world or some other big site to turn on this logging and share it so we can try to better reproduce the performance overloads on development/testing systems. Thank you.

I thought some people were out there in June creating stress-testing scripts, but I haven't seen anything materializing/showing results in recent weeks?

I think it would be useful to have an API client that establishes some baseline performance number that can be run before a new release of Lemmy and at least ensure there is no performance regression?

The biggest problem I have had since day 1 is not being able to reproduce the data that lemmy.ml has inside. There is a lot of older content stored that does not get replicated, etc.

The site_aggregates UPDATE statement lacking a WHERE clause and hitting 1500 rows (number of known Lemmy instances) of data instead of 1 row is exactly the kind of data-centered problem that has slipped through the cracks. That was generating a ton of extra PostgreSQL I/O for every new comment and post from a local user.

The difficult things to take on:

1. Simulating 200 instances instead of just 5 that the current API testing code does. First, just to have 200 rows in many of the instance-specific tables so that local = false API calls are better exercised. And probably about 25 of those instances have a large number of remote subscribers to communities.

2. async federation testing. The API testing in lemmy right now does immediate delivery with the API call so you don't get to find out the tricky cases of servers being unreachable.

3. Bulk loading of data. On one hand it is good to exercise the API by inserting posts and comments one at a time, but maybe loading data directly into the PostgreSQL backend would speed up development and testing?

4. The impact of scheduled jobs such as updates to certain aggregate data and post ranking for sorting. We may want to add special API feature for testing code to trigger these on-demand to stress test that concurrency with PostgreSQL isn't running into overloads.

5. Historically, there have been changes to the PostgreSQL table layout and indexes (schema) with new versions of Lemmy, which can take significant time to execute on a production server with existing data. Some kind of expectation for server operators to know how long an upgrade can take to modify data.

6. Searching on communities, posts, comments with significant amounts of data in PostgreSQL. Scanning content of large numbers of posts and comments can be done by users at any time.

7. non-Lemmy federated content in database. Possible performance and code behavior that arises from Mastodon and other non-Lemmy interactions.

I don't think it would be a big deal if the test takes 30 minutes or even longer to run.

And I'll go out and say it: Is a large Lemmy server willing to offer a copy of their database for performance troubleshooting and testing? Lemmy.ca cloned their database last Sunday which lead to the discovery of site_aggregates UPDATE without WHERE problem. Maybe we can create a procedure of how to remove private messages and get a dump once a month from a big server to analyze possible causes of PostgreSQL overloads? This may be a faster path than building up from-scratch with new testing logic.

IIRC, it was lemmy.ca full copy of live data that was used (copy made on development system, not live server - if I'm following). Shared Saturday July 22 on GitHub was this procedure:

...

Notable is the AUTO_EXPLAIN SQL activation statements:

LOAD 'auto_explain';
SET auto_explain.log_min_duration = 0;
SET auto_explain.log_analyze = true;
SET auto_explain.log_nested_statements = true;

This technique would be of great use for developers doing changes and study of PostgreSQL activity. Thank you!

Right now querying posts has logic like this:

WHERE (((((((((("community"."removed" = $9) AND ("community"."deleted" = $10)) AND ("post"."removed" = $11)) AND ("post"."deleted" = $12)) AND (("community"."hidden" = $13)

Note that a community can be hidden or deleted, separate fields. And it also has logic to see if the creator of the post is banned in the community:

LEFT OUTER JOIN "community_person_ban" ON (("post"."community_id" = "community_person_ban"."community_id") AND ("community_person_ban"."person_id" = "post"."creator_id"))

And there is both a deleted boolean (end-user delete) and removed boolean (moderator removed) on a post.

Much of this also applies to comments. Which are also owned by the post, which are also owned by the community.

in Communities create community/edit community there is a SiteLanguage::read with no site_id, should that call to read have site_id = 1?

For reference, on my production instance my site_language table has 198460 rows and my site table has 1503 rows. Average of 132 languages per site. counts: https://lemmyadmin.bulletintree.com/query/pgcounts?output=table

A general description of the proposed change and reasoning behind it is on GitHub: https://github.com/LemmyNet/lemmy/issues/3697

Linear execution of these massive changes to votes/comments/posts with concurrency awareness. Also adds a layer of social awareness, the impact on a community when a bunch of content is black-holed.

An entire site federation delete / dead server - also would fall under this umbrella of mass data change with a potential for new content ownership/etc.

Over a short period of time, this is my incoming federation activity for new comments. pg_stat_statements output being show. It is interesting to note these two INSERT statements on comments differ only in the DEFAULT value of language column. Also note the average execution times is way higher (4.3 vs. 1.28) when the language value is set, I assume due to INDEX updates on the column? Or possibly a TRIGGER?

About half of the comments coming in from other servers have default value.

WRITES are heavy, even if it is an INDEX that has to be revised. So INSERT and UPDATE statements are important to scrutinize.

Given how frequent these records are created, every vote by a user, I think it is important to study and review how it works.

The current design of lemmy_server 0.18.3 is to issue a SQL DELETE before (almost?) every INSERT of a new vote. The INSERT already has an UPDATE clause on it.

This is one of the few places in Lemmy that a SQL DELETE statement actually takes place. We have to be careful triggers are not firing multiple times, such as decreasing the vote to then immediately have it increase with the INSERT statement that comes later.

For insert of a comment, Lemmy doesn't seem to routinely run a DELETE before the INSERT. So why was this design chosen for votes? Likely the reason is because a user can "undo" a vote and have the record of them ever voting in the database removed. Is that the actual behavior in testing?

pg_stat_statements from an instance doing almost entirely incoming federation activity of post/comments from other instances:

• DELETE FROM "comment_like" WHERE (("comment_like"."comment_id" = $1) AND ("comment_like"."person_id" = $2)) executed 14736 times, with 607 matching records.

• INSERT INTO "comment_like" ("person_id", "comment_id", "post_id", "score") VALUES ($1, $2, $3, $4) ON CONFLICT ("comment_id", "person_id") DO UPDATE SET "person_id" = $5, "comment_id" = $6, "post_id" = $7, "score" = $8 RETURNING "comment_like"."id", "comment_like"."person_id", "comment_like"."comment_id", "comment_like"."post_id", "comment_like"."score", "comment_like"."published" executed 15883 times - each time transacting.

• update comment_aggregates ca set score = score + NEW.score, upvotes = case when NEW.score = 1 then upvotes + 1 else upvotes end, downvotes = case when NEW.score = -1 then downvotes + 1 else downvotes end where ca.comment_id = NEW.comment_id TRIGGER FUNCTION update executing 15692 times.

• update person_aggregates ua set comment_score = comment_score + NEW.score from comment c where ua.person_id = c.creator_id and c.id = NEW.comment_id TRIGGER FUNCTION update, same executions as previous.

There is some understanding to gain by the count of executions not being equal.

Details here: https://github.com/LemmyNet/lemmy/issues/3165

This will VASTLY decrease the server load of I/O for PostgreSQL, as this mistaken code is doing writes of ~1700 rows (each known Lemmy instance in the database) on every single comment & post creation. This creates record-locking issues given it is writes, which are harsh on the system. Once this is fixed, some site operators will be able to downgrade their hardware! ;)

Improving Beehaw

BLUF: The operations team at Beehaw has worked to increase site performance and uptime. This includes proactive monitoring to prevent problems from escalating and planning for future likely events.

Problem: Emails only sent to approved users, not denials; denied users can't reapply with the same username

• Solution: Made it so denied users get emails and their usernames freed up to re-use

Details:

• Disabled Docker postfix container; Lemmy runs on a Linux host that can use postfix itself, without any overhead

• Modified various postfix components to accept localhost (same system) email traffic only

• Created two different scripts to:

  • Check the Lemmy database once in while, for denied users, send them an email and delete the user from the database
    • User can use the same username to register again!
  • Send out emails to those users (and also, make the other Lemmy emails look nicer)

• Sending so many emails from our provider caused the emails to end up in spam!! We had to change a bit of the outgoing flow

  • DKIM and SPF setup
  • Changed outgoing emails to relay through Mailgun instead of through our VPS

• Configure Lemmy containers to use the host postfix as mail transport

All is well?

Problem: NO file level backups, only full image snapshots

• Solution: Procured backup storage (Backblaze B2) and setup system backups, tested restoration (successfully)

Details:

• Requested Funds from Beehaw org to be spent on purchase of cloud based storage, B2 - approved (thank you for the donations)

• Installed and configured restic encrypted backups of key system files -> b2 'offsite'. This means, even the data from Beehaw that is saved there, is encrypted and no one else can read that information

• Verified scheduled backups are being run every day, to b2. Important information such as the Lemmy volumes, pictures, configurations for various services, and a database dump are included in such

• Verified restoration works! Had a small issue with the pictrs migration to object storage (b2). Restored the entire pictrs volume from restic b2 backup successfully. Backups work!

sorry for that downtime, but hey.. it worked

Problem: No metrics/monitoring; what do we focus on to fix?

• Solution: Configured external system monitoring via external SNMP, internal monitoring for services/scripts

Details:

• Using an existing self-hosted Network Monitoring Solution (thus, no cost), established monitoring of Beehaw.org systems via SNMP
• This gives us important metrics such as network bandwidth usage, Memory, and CPU usage tracking down to which process are using the most, parsing system event logs and tracking disk IO/Usage
• Host based monitoring that is configured to perform actions for known error occurrences and attempts to automatically resolve them. Such as Lemmy app; crashing; again
• Alerting for unexpected events or prolonged outages. Spams the crap out of @admin and @Lionir. They love me
• Database level tracking for 'expensive' queries to know where the time and effort is spent for Lemmy. Helps us to report these issues to the developers and get it fixed.

With this information we've determined the areas to focus on are database performance and storage concerns. We'll be moving our image storage to a CDN if possible to help with bandwidth and storage costs.

Peace of mind, and let the poor admins sleep!

Problem: Lemmy is really slow and more resources for it are REALLY expensive

• Solution: Based on metrics (see above), tuned and configured various applications to improve performance and uptime

Details:

• I know it doesn't seem like it, but really, uptime has been better with a few exceptions
• Modified NGINX (web server) to cache items and load balance between UI instances (currently running 2 lemmy-ui containers)
• Setup frontend varnish cache to decrease backend (Lemmy/DB) load. Save images and other content before hitting the webserver; saves on CPU resources and connections, but no savings to bandwidth cost
• Artificially restricting resource usage (memory, CPU) to prove that Lemmy can run on less hardware without a ton of problems. Need to reduce the cost of running Beehaw

THE DATABASE

This gets it's own section. Look, the largest issue with Lemmy performance is currently the database. We've spent a lot of time attempting to track down why and what it is, and then fixing what we reliably can. However, none of us are rust developers or database admins. We know where Lemmy spends its time in the DB but not why and really don't know how to fix it in the code. If you've complained about why is Lemmy/Beehaw so slow this is it; this is the reason.

So since I can't code rust, what do we do? Fix it where we can! Postgresql server setting tuning and changes. Changed the following items in postgresql to give better performance based on our load and hardware:

 huge_pages = on # requires sysctl.conf changes and a system reboot
 shared_buffers = 2GB
 max_connections = 150
 work_mem = 3MB
 maintenance_work_mem = 256MB
 temp_file_limit = 4GB
 min_wal_size = 1GB
 max_wal_size = 4GB
 effective_cache_size = 3GB
 random_page_cost = 1.2
 wal_buffers = 16MB
 bgwriter_delay = 100ms
 bgwriter_lru_maxpages = 150
 effective_io_concurrency = 200
 max_worker_processes = 4 
 max_parallel_workers_per_gather = 2
 max_parallel_maintenance_workers = 2
 max_parallel_workers = 6
 synchronous_commit = off  	
 shared_preload_libraries = 'pg_stat_statements'
 pg_stat_statements.track = all

Now I'm not saying all of these had an affect, or even a cumulative affect; just the values we've changed. Be sure to use your own system values and not copy the above. The three largest changes I'd say are key to do are synchronous_commit = off, huge_pages = on and work_mem = 3MB. This article may help you understand a few of those changes.

With these changes, the database seems to be working a damn sight better even under heavier loads. There are still a lot of inefficiencies that can be fixed with the Lemmy app for these queries. A user phiresky has made some huge improvements there and we're hoping to see those pulled into main Lemmy on the next full release.

Problem: Lemmy errors aren't helpful and sometimes don't even reach the user (UI)

• Solution: Make our own UI with ~~blackjack and hookers~~ propagation for backend Lemmy errors. Some of these fixes have been merged into Lemmy main codebase

Details

• Yeah, we did that. Including some other UI niceties. Main thing is, you need to pull in the lemmy-ui code make your changes locally, and then use that custom image as your UI for docker
• Made some changes to a custom lemmy-ui image such as handling a few JSON parsed error better, improving feedback given to the user
• Remove and/or move some elements around, change the CSS spacing
• Change the node server to listen to system signals sent to it, such as a graceful docker restart
• Other minor changes to assist caching, changed the container image to Debian based instead of Alpine (reducing crashes)

The end?

No, not by far. But I am about to hit the character limit for Lemmy posts. There have been many other changes and additions to Beehaw operations, these are but a few of the key changes. Sharing with the broader community so those of you also running Lemmy, can see if these changes help you too. Ask questions and I'll discuss and answer what I can; no secret sauce or passwords though; I'm not ChatGPT.

Shout out to @Lionir@beehaw.org , @Helix@beehaw.org and @admin@beehaw.org for continuing to work with me to keep Beehaw running smoothly.

Thanks all you Beeple, for being here and putting up with our growing pains!

lemmy.world announcing to public that this is installed on their production server: https://lemmy.world/post/1061471

It looks like the lack of persistent storage for the federated activity queue is leading to instances running out of memory in a matter of hours. See my comment for more details.

Furthermore, this leads to data loss, since there is no other consistency mechanism. I think it might be a high priority issue, taking into account the current momentum behind growth of Lemmy...

https://docs.rs/moka/latest/moka/

I spent several hours tracing in production (updating the code a dozen times with extra logging) to identify the actual path the lemmy_server code uses for outbound federation of votes to subscribed servers.

Major popular servers, Beehaw, Leemy.world, Lemmy.ml - have a large number of instance servers subscribing to their communities to get copies of every post/comment. Comment votes/likes are the most common activity, and it is proposed that during the PERFORMANCE CRISIS that outbound vote/like sharing be turned off by these overwhelmed servers.

pull request for draft:

https://github.com/LemmyNet/lemmy/compare/main...RocketDerp:lemmy_comment_votes_nofed1:no_federation_of_votes_outbound0

EDIT: LEMMY_SKIP_FEDERATE_VOTES environment variable

Grep the lemmy server code for "= LocalSite::read" - and I find that even for a single vote by an end-user, it is doing an SQL query to the local site settings to see if downvotes are disabled.

Can some Rust programmers chime in here? Can we cache this in RAM and not fetch from SQL every time?

PostgreSQL is telling me that the 2nd most run query on my system, which is receiving incoming federation post/comment/votes, is this:

SELECT "local_site"."id", "local_site"."site_id", "local_site"."site_setup", "local_site"."enable_downvotes", "local_site"."enable_nsfw", "local_site"."community_creation_admin_only", "local_site"."require_email_verification", "local_site"."application_question", "local_site"."private_instance", "local_site"."default_theme", "local_site"."default_post_listing_type", "local_site"."legal_information", "local_site"."hide_modlog_mod_names", "local_site"."application_email_admins", "local_site"."slur_filter_regex", "local_site"."actor_name_max_length", "local_site"."federation_enabled", "local_site"."captcha_enabled", "local_site"."captcha_difficulty", "local_site"."published", "local_site"."updated", "local_site"."registration_mode", "local_site"."reports_email_admins" FROM "local_site" LIMIT $1

Has anyone come up with some admin tools to display anything helpful regarding servers like

Community federation status last sync pass fail etc

General db stats size etc

Specifically the database backend. Is the polling for notifications causing more database load? From my personal testing, lemmy.ml has the same performance problems with 0.17.4 that it does with 0.18.1 and I haven't seen anything in the code changes that are that significant with database.

Heavy loads have been brought up several times, major social events where people flock to social media. Such as a terrorist bombing, submarine sinking, earthquake, nuclear meltdown, celebrity airplane crash, etc.

Low-budget hosting to compete with the tech giants is also a general concern for the project. Trying not to have a server that uses tons of expensive resources during some peak usage.

Load shedding and self-tuning within the code and for SysOps to have some idea if their Lemmy server is nearing overload thresholds.

See comments: