Messages in this thread | | | Date | Sun, 28 Jul 2013 13:02:12 +0200 | From | Marco Stornelli <> | Subject | Re: [PATCH RFC] pram: persistent over-kexec memory file system |
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Il 28/07/2013 12:05, Vladimir Davydov ha scritto: > On 07/27/2013 09:37 PM, Marco Stornelli wrote: >> Il 27/07/2013 19:35, Vladimir Davydov ha scritto: >>> On 07/27/2013 07:41 PM, Marco Stornelli wrote: >>>> Il 26/07/2013 14:29, Vladimir Davydov ha scritto: >>>>> Hi, >>>>> >>>>> We want to propose a way to upgrade a kernel on a machine without >>>>> restarting all the user-space services. This is to be done with CRIU >>>>> project, but we need help from the kernel to preserve some data in >>>>> memory while doing kexec. >>>>> >>>>> The key point of our implementation is leaving process memory in-place >>>>> during reboot. This should eliminate most io operations the services >>>>> would produce during initialization. To achieve this, we have >>>>> implemented a pseudo file system that preserves its content during >>>>> kexec. We propose saving CRIU dump files to this file system, >>>>> kexec'ing >>>>> and then restoring the processes in the newly booted kernel. >>>>> >>>> >>>> http://pramfs.sourceforge.net/ >>> >>> AFAIU it's a bit different thing: PRAMFS as well as pstore, which has >>> already been merged, requires hardware support for over-reboot >>> persistency, so called non-volatile RAM, i.e. RAM which is not directly >>> accessible and so is not used by the kernel. On the contrary, what we'd >>> like to have is preserving usual RAM on kexec. It is possible, because >>> RAM is not reset during kexec. This would allow leaving applications >>> working set as well as filesystem caches in place, speeding the reboot >>> process as a whole and reducing the downtime significantly. >>> >>> Thanks. >> >> Actually not. You can use normal system RAM reserved at boot with mem >> parameter without any kernel change. Until an hard reset happens, that >> area will be "persistent". > > Thank you, we'll look at PRAMFS closer, but right now, after trying it I > have a couple of concerns I'd appreciate if you could clarify: > > 1) As you advised, I tried to reserve a range of memory (passing > memmap=4G$4G at boot) and mounted PRAMFS using the following options: > > # mount -t pramfs -o physaddr=0x100000000,init=4G,bs=4096 none /mnt/pramfs > > And it turned out that PRAMFS is very slow as compared to ramfs: > > # dd if=/dev/zero of=/mnt/pramfs if=/dev/zero of=/mnt/pramfs/dummy > bs=4096 count=$[100*1024] > 102400+0 records in > 102400+0 records out > 419430400 bytes (419 MB) copied, 9.23498 s, 45.4 MB/s > # dd if=/dev/zero of=/mnt/pramfs if=/dev/zero of=/mnt/pramfs/dummy > bs=4096 count=$[100*1024] conv=notrunc > 102400+0 records in > 102400+0 records out > 419430400 bytes (419 MB) copied, 3.04692 s, 138 MB/s > > We need it to be as fast as usual RAM, because otherwise the benefit of > it over hdd disappears. So before diving into the code, I'd like to ask > you if it's intrinsic to PRAMFS, or can it be fixed? Or, perhaps, I used > wrong mount/boot/config options (btw, I enabled only CONFIG_PRAMFS)? >
In x86 you should have the write protection enabled. Turn it off or mount it with noprotect option.
> 2) To enable saving application dump files in memory using PRAMFS, one > should reserve half of RAM for it. That's too expensive. While with > ramfs, once SPLICE_F_MOVE flag is implemented, one could move anonymous > memory pages to ramfs page cache and after kexec move it back so that > almost no extra memory space costs would be required. Of course, > SPLICE_F_MOVE is to be yet implemented, but with PRAMFS significant > memory costs are inevitable... or am I wrong? > > Thanks.
From this point of view you are right. Pramfs (or other solution like that) are out of page cache, so you can't do any memory transfer. It's like to have a disk but it's actually a separate piece of RAM. We could talk about it again when this kind of implementation will be done.
Marco
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