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A group scheduling demonstration

A group scheduling demonstration

Posted Mar 28, 2011 21:56 UTC (Mon) by Duncan (guest, #6647)
In reply to: A group scheduling demonstration by kabloom
Parent article: A group scheduling demonstration

There are two problems with disabling swap entirely, however.

1) Suspend-to-disk (aka hibernate) uses swap-space for the suspend image. No swap-space, no hibernate.

This can be worked around by enabling the desired suspend image swapspace in the hibernate script itself. I use a custom hibernate script here that does that, with the partition it uses and whether it enables it or not part of the configurables. Of course if it enabled it, it disables it after resume, too.

2) In some cases, disabling swap at least /used/ to disable memory transfer between various memory zones, etc. It's not as critical for 64-bit, but for 32-bit especially, it's quite possible to have a shortage of memory within one zone while the system as a whole still has plenty of memory, so having the flexibility to transfer memory between zones, where the memory ranges overlap and it can be used for either zone, can be very important.

Meanwhile, this won't work for everyone, but on machines with multiple storage devices (normally disks, but with SSD these days...), setting up multiple swap partitions, one per device, and specifically setting equivalent swap priority on all partitions, directs the kernel to stripe swap across all partitions at that priority, effectively RAID-0-ing them. I have four SATA drives here with most of the system in RAID-1, but have arranged swap as above, across all four devices. I can be a half-gig or more into swap before I really start to notice it, and at one point I had a runaway process that I let eat all 8 gigs of physical memory plus 15+ gigs of the 16 gigs of swap (4-gig each device, as they were laid out identically and that gave me a single partition of 4 gigs, half the size of memory, for the suspend image, as that was the biggest suspend image allowed), before I killed it, just because it was fascinating to watch. After a couple gigs of swap or so, there was a definite drag on responsiveness, but the system never did totally lock up, and when I realized what was happening, I opened a shell and had the kill command all typed in ready to hit enter, before it hit double-digit gigs of swap. After that it was just watch it happen and wait until it got as close as I dared to the limit, before I hit enter and killed the offender.

Also, while everybody else writing about it seems to recommend reducing vm.swappiness to something like 20, from the default 60, on the above striped swap config, I invert that, running vm.swappiness=100, as I'd rather dump into the 4-way-striped swap than dump hard-earned RAID-1 cache. Works great here! =:^)

Duncan


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A group scheduling demonstration

Posted Mar 29, 2011 7:35 UTC (Tue) by rvfh (subscriber, #31018) [Link]

> 1) Suspend-to-disk (aka hibernate) uses swap-space for the suspend image. No swap-space, no hibernate.

According to the kernel documentation, you can set up a swap file for this. Not tried yet though...


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