From c0265342bff4fcaa2cdf13f4596244c18d4a7ae5 Mon Sep 17 00:00:00 2001 From: Minchan Kim Date: Thu, 7 Jun 2018 17:05:49 -0700 Subject: zram: introduce zram memory tracking zRam as swap is useful for small memory device. However, swap means those pages on zram are mostly cold pages due to VM's LRU algorithm. Especially, once init data for application are touched for launching, they tend to be not accessed any more and finally swapped out. zRAM can store such cold pages as compressed form but it's pointless to keep in memory. Better idea is app developers free them directly rather than remaining them on heap. This patch tell us last access time of each block of zram via "cat /sys/kernel/debug/zram/zram0/block_state". The output is as follows, 300 75.033841 .wh 301 63.806904 s.. 302 63.806919 ..h First column is zram's block index and 3rh one represents symbol (s: same page w: written page to backing store h: huge page) of the block state. Second column represents usec time unit of the block was last accessed. So above example means the 300th block is accessed at 75.033851 second and it was huge so it was written to the backing store. Admin can leverage this information to catch cold|incompressible pages of process with *pagemap* once part of heaps are swapped out. I used the feature a few years ago to find memory hoggers in userspace to notify them what memory they have wasted without touch for a long time. With it, they could reduce unnecessary memory space. However, at that time, I hacked up zram for the feature but now I need the feature again so I decided it would be better to upstream rather than keeping it alone. I hope I submit the userspace tool to use the feature soon. [akpm@linux-foundation.org: fix i386 printk warning] [minchan@kernel.org: use ktime_get_boottime() instead of sched_clock()] Link: http://lkml.kernel.org/r/20180420063525.GA253739@rodete-desktop-imager.corp.google.com [akpm@linux-foundation.org: documentation tweak] [akpm@linux-foundation.org: fix i386 printk warning] [minchan@kernel.org: fix compile warning] Link: http://lkml.kernel.org/r/20180508104849.GA8209@rodete-desktop-imager.corp.google.com [rdunlap@infradead.org: fix printk formats] Link: http://lkml.kernel.org/r/3652ccb1-96ef-0b0b-05d1-f661d7733dcc@infradead.org Link: http://lkml.kernel.org/r/20180416090946.63057-5-minchan@kernel.org Signed-off-by: Minchan Kim Signed-off-by: Randy Dunlap Reviewed-by: Sergey Senozhatsky Acked-by: Greg Kroah-Hartman Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- Documentation/blockdev/zram.txt | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) (limited to 'Documentation') diff --git a/Documentation/blockdev/zram.txt b/Documentation/blockdev/zram.txt index 78db38d02bc9..875b2b56b87f 100644 --- a/Documentation/blockdev/zram.txt +++ b/Documentation/blockdev/zram.txt @@ -243,5 +243,29 @@ to backing storage rather than keeping it in memory. User should set up backing device via /sys/block/zramX/backing_dev before disksize setting. += memory tracking + +With CONFIG_ZRAM_MEMORY_TRACKING, user can know information of the +zram block. It could be useful to catch cold or incompressible +pages of the process with*pagemap. +If you enable the feature, you could see block state via +/sys/kernel/debug/zram/zram0/block_state". The output is as follows, + + 300 75.033841 .wh + 301 63.806904 s.. + 302 63.806919 ..h + +First column is zram's block index. +Second column is access time since the system was booted +Third column is state of the block. +(s: same page +w: written page to backing store +h: huge page) + +First line of above example says 300th block is accessed at 75.033841sec +and the block's state is huge so it is written back to the backing +storage. It's a debugging feature so anyone shouldn't rely on it to work +properly. + Nitin Gupta ngupta@vflare.org -- cgit v1.2.3