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virtual-memorymemory-pressure
Tim Stewart
Tim Stewart

Reputation: 5500

Understanding Negative Virtual Memory Pressure

I was re-reading Poul-Henning Kamp's paper entitled, "You're Doing It Wrong" and one of the diagrams confused me.

The x-axis of Figure 1 is labeled as "VM pressure in megabytes". The author clarifies the x-axis as being "measured in the amount of address space not resident in primary memory, because the kernel paged it out to secondary storage".

I can understand zero MB of VM pressure (all of the address space is resident in primary memory).

I can understand a positive VM pressure but I'm having a tough time picturing what negative 8 megabytes of VM pressure looks like (see the left of the x-axis of Figure 1). Putting negative 8 in the author's description leaves me with, "- 8 MB of address space not resident in primary memory". That doesn't make sense to me.

If I just conclude that the author accidentally negated positive numbers, the chart makes more sense but I'm not ready to conclude that the author has made the mistake. It's more likely that I have. But then as the pressure decreases, the runtime increases? That sounds counterintuitive.

I'm also not sure why there is a drastic change to the curves around -8 MB of VM memory pressure.

Thanks in advance!

Upvotes: 2

Views: 488

Answers (1)

Basilevs
Basilevs

Reputation: 23921

Read "measured in the difference between amount of address space resident in primary memory and total required amount".

Word "not" somehow represents that minus sign.

Upvotes: 1

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