Hi Minchan, On 08/21/2013 02:16 PM, Minchan Kim wrote: > It's 7th trial of zram/zsmalloc promotion. > I rewrote cover-letter totally based on previous discussion. > > The main reason to prevent zram promotion was no review of > zsmalloc part while Jens, block maintainer, already acked > zram part. > > At that time, zsmalloc was used for zram, zcache and zswap so > everybody wanted to make it general and at last, Mel reviewed it > when zswap was submitted to merge mainline a few month ago. > Most of review was related to zswap writeback mechanism which > can pageout compressed page in memory into real swap storage > in runtime and the conclusion was that zsmalloc isn't good for > zswap writeback so zswap borrowed zbud allocator from zcache to > replace zsmalloc. The zbud is bad for memory compression ratio(2) > but it's very predictable behavior because we can expect a zpage > includes just two pages as maximum. Other reviews were not major. > http://lkml.indiana.edu/hypermail/linux/kernel/1304.1/04334.html > > Zcache doesn't use zsmalloc either so zsmalloc's user is only > zram now so this patchset moves it into zsmalloc directory. > Recently, Bob tried to move zsmalloc under mm directory to unify > zram and zswap with adding pseudo block device in zswap(It's > very weired to me) but he was simple ignoring zram's block device > (a.k.a zram-blk) feature and considered only swap usecase of zram, > in turn, it lose zram's good concept. > Yes, I didn't notice the feature that zram can be used as a normal block device. > Mel raised an another issue in v6, "maintainance headache". > He claimed zswap and zram has a similar goal that is to compresss > swap pages so if we promote zram, maintainance headache happens > sometime by diverging implementaion between zswap and zram > so that he want to unify zram and zswap. For it, he want zswap > to implement pseudo block device like Bob did to emulate zram so > zswap can have an advantage of writeback as well as zram's benefit. If consider zram as a swap device only, I still think it's better to add a pseudo block device to zswap and just disable the writeback of zswap. But I have no idea of zram's block device feature. > But I wonder frontswap-based zswap's writeback is really good > approach for writeback POV. I think that problem isn't only > specific for zswap. If we want to configure multiple swap hierarchy > with various speed device such as RAM, NVRAM, SSD, eMMC, NAS etc, > it would be a general problem. So we should think of more general > approach. At a glance, I can see two approach. > > First, VM could be aware of heterogeneous swap configuration > so it could aim for being able to configure cache hierarchy > among swap devices. It may need indirction layer on swap, which > was already talked about that way so VM can migrate a block from > A to B easily. It will support various configuration with VM's > hints, maybe, in future. > http://lkml.indiana.edu/hypermail/linux/kernel/1203.3/03812.html > > Second, as more practical solution, we could use device mapper like > dm-cache(https://lwn.net/Articles/540996/), which makes it very > flexible. Now, it supports various configruation and cache policy > (block size, writeback/writethrough, LRU, MFU although MQ is merged > now) so it would be good fit for our purpose. Even, it can make zram > support writeback. I tested it following as following scenario > in KVM 4 CPU, 1G DRAM with background 800M memory hogger, which is > allocates random data up to 800M. > > 1) zram swap disk 1G, untar kernel.tgz to tmpfs, build -j 4 > Fail to untar due to shortage of memory space by tmpfs default size limit > > 2) zram swap disk 1G, untar kernel.tgz to ext2 on zram-blk, build -j 4 > OOM happens while building the kernel but it untar successfully > on ext2 based on zram-blk. The reason OOM happend is zram can not find > free pages from main memory to store swap out pages although empty > swap space is still enough. > > 3) dm-cache swap disk 1G, untar kernel.tgz to ext2 on zram-blk, build -j 4 > dmcache consists of zram-meta 10M, zram-cache 1G and real swap storage 1G > No OOM happens and successfully building done. > > Above tests proves zram can support writeback into real swap storage > so that zram-cache can always have a free space. If necessary, we could > add new plugin in dm-cache. I see It's really flexible and well-layered > architecure so zram-blk's concept is good for us and it has lots of > potential to be enhanced by MM/FS/Block developers. > That's an exciting direction! -- Regards, -Bob -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>