––thursday #5: diagnosing high readahead

Having readahead set too high can slow your database to a crawl. This post discusses why that is and how you can diagnose it.

The #1 sign that readahead is too high is that MongoDB isn’t using as much RAM as it should be. If you’re running Mongo Monitoring Service (MMS), take a look at the “resident” size on the “memory” chart. Resident memory can be thought of as “the amount of space MongoDB ‘owns’ in RAM.” Therefore, if MongoDB is the only thing running on a machine, we want resident size to be as high as possible. On the chart below, resident is ~3GB:

Is 3GB good or bad? Well, it depends on the machine. If the machine only has 3.5GB of RAM, I’d be pretty happy with 3GB resident. However, if the machine has, say, 15GB of RAM, then we’d like at least 15GB of the data to be in there (the “mapped” field is (sort of) data size, so I’m assuming we have 60GB of data).

Assuming we’re accessing a lot of this data, we’d expect MongoDB’s resident set size to be 15GB, but it’s only 3GB. If we try turning down readahead and the resident size jumps to 15GB and our app starts going faster. But why is this?

Let’s take an example: suppose all of our docs are 512 bytes in size (readahead is set in 512-byte increments, called sectors, so 1 doc = 1 sector makes the math easier). If we have 60GB of data then we have ~120 million documents (60GB of data/(512 bytes/doc)). The 15GB of RAM on this machine should be able to hold ~30 million documents.

Our application accesses documents randomly across our data set, so we’d expect MongoDB to eventually “own” (have resident) all 15GB of RAM, as 1) it’s the only thing running and 2) it’ll eventually fetch at least 15GB of the data.

Now, let’s set our readahead to 100 (100 512-byte sectors, aka 100 documents): blockdev --set-ra 100. What happens when we run our application?

Picture our disk as looking like this, where each o is a document:

...
ooooooooooooooooooooooooo
ooooooooooooooooooooooooo
ooooooooooooooooooooooooo
ooooooooooooooooooooooooo
ooooooooooooooooooooooooo
ooooooooooooooooooooooooo
ooooooooooooooooooooooooo
ooooooooooooooooooooooooo
... // keep going for millions more o's

Let’s say our app requests a document. We’ll mark it with “x” to show that the OS has pulled it into memory:

...
ooooooooooooooooooooooooo
ooooxoooooooooooooooooooo
ooooooooooooooooooooooooo
ooooooooooooooooooooooooo
ooooooooooooooooooooooooo
ooooooooooooooooooooooooo
ooooooooooooooooooooooooo
ooooooooooooooooooooooooo
...

See it on the third line there? But that’s not the only doc that’s pulled into memory: readahead is set to 100 so the next 99 documents are pulled into memory, too:

...
ooooooooooooooooooooooooo
ooooxxxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxx
xxxxxxxxxxxxxxxxxxxxxxxxx
xxxxooooooooooooooooooooo
ooooooooooooooooooooooooo
ooooooooooooooooooooooooo
...

Now we have 100 docs in memory, but remember that our application is accessing documents randomly: the likelihood of the next document we access is in that block of 100 docs is almost nil. At this point, there’s 50KB of data in RAM (512 bytes * 100 docs = 51,200 bytes) and MongoDB’s resident size has only increase by 512 bytes (1 doc).

Our app will keep bouncing around the disk, reading docs from here and there and filing up memory with docs MongoDB never asked for until RAM is completely full of junk that’s never been used. Then, it’ll start evicting things to make room for new junk as our app continues to make requests.

Working this out, there’s a 25% chance of our app requesting a doc that’s already in memory, so 75% of the requests are going to go to disk. Say we’re doing 2 requests a sec. Then 1 hour of requests is 2 requests * 3600 seconds/hour = 7200 requests, 4800 of which are going to disk (.75 * 7200). If each request pulls back 50KB, that’s 240MB read from disk/hour. If we set readahead to 0, we’ll have 2MB read from disk/hour.

Which brings us to the next symptom of a too-high readahead: unexpectedly high disk IO. Because most of the data we want isn’t in memory, we keep having to go to disk, dragging shopping-carts full of junk into RAM, perpetuating the high disk io/low resident mem cycle.

The general takeaway is that a DB is not a “normal” workload for an OS. The default settings may screw you over.