Messages in this thread | | | Date | Wed, 12 Jun 2013 15:42:15 -0500 | From | Rob Landley <> | Subject | Re: Slow swap-in with SSD |
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On 06/11/2013 09:34:36 AM, Martin Steigerwald wrote: > Hi! > > Using Linux 3.10-rc5 on an ThinkPad T520 with Intel Sandybridge > i5-2620M, > 8 GiB RAM and Intel SSD 320. Currently I have Zcache enabled to test > the > effects of it but I observed similar figures on kernels without > Zcache. > > If I let the kernel swap out for example with > > stress -m 1 --vm-keep --vm-bytes 5G > > or so, then swapping out is pretty fast, I have seen values around > 100-200 MiB/s > > But on issuing a swapoff command to swap stuff in again, the swap in > is > abysmally slow, just a few MiB/s (see below). > > I wonder why is that so? The SSD is basically idling around on > swap-in.
Transaction granularity. Swapping out can queue up large batches of pages because you can queue up more outgoing pages while the others are still writing. Swapping _in_ you don't know what you need next until you resume the process, so you fault in 4k, schedule DMA, resume the process when it completes, fault on the next page, schedule more DMA, rinse repeat. Programs don't really execute linearly, so you wind up with round trip latency to and from device each time.
The problem with doing readahead on swapin is that programs jump around randomly calling a function here and a function there, so you dunno which other pages it'll need until it requests them. (Speculatively faulting in pages when the system is starved of memory usually just makes the memory shortage worse. This code only runs when there's a shortage of physical pages.)
Having an ssd just exacerbates the problem, because with the device itself sped up the round trip latency from all the tiny transactions comes to dominate.
Rob
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