Introducing Aggressive Low Memory Booster [1]

[PINTU KUMAR <pintu_agarwal@yahoo.com>]

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Hi,

Here I am trying to introduce a new feature in kernel called "Aggressive Low Memory Booster".
The main advantage of this will be to boost the available free memory of the system to "certain level" during extremely low memory condition.

Please provide your comments to improve further.
Can it be used along with vmpressure_fd ???


It can be invoked as follows:
    a) Automatically by kernel memory management when the memory threshold falls below 10MB.
    b) From user space program/scripts by passing the "required amount of memory to be reclaimed".
    Example: echo 100 > /dev/shrinkmem
    c) using sys interface - /sys/kernel/debug/shrinkallmem
    d) using an ioctl call and returning number of pages reclaimed.
    e) using a new system call - shrinkallmem(&nrpages);
    f) During CMA to reclaim and shrink a specific CMA regions.


I have developed a kernel module to verify the (b) part.

Here is the snapshot of the write call:
+static ssize_t shrinkmem_write(struct file *file, const char *buff,
+                                size_t length, loff_t *pos)
+{
+        int ret = -1;
+        unsigned long memsize = 0;
+        unsigned long nr_reclaim = 0;
+        unsigned long pages = 0;
+        ret = kstrtoul_from_user(buff, length, 0, &memsize);
+        if (ret < 0) {
+                printk(KERN_ERR "[SHRINKMEM]: kstrtoul_from_user: Failed !\n");
+                return -1;
+        }
+        printk(KERN_INFO "[SHRINKMEM]: memsize(in MB) = %ld\n",
+                                (unsigned long)memsize);
+        memsize = memsize*(1024UL*1024UL);
+        nr_reclaim = memsize / PAGE_SIZE;
+        pages = shrink_all_memory(nr_reclaim);
+        printk(KERN_INFO "<SHRINKMEM>: Number of Pages Freed: %lu\n", pages);
+        return pages;
+}
Please note: This requires CONFIG_HIBERNATION to be permanently enabled in the kernel.


Several experiments have been performed on Ubuntu(kernel 3.3) to verify it under low memory conditions.

Following are some results obtained:
-------------------------------------

Node 0, zone      DMA    290    115      0      0      0      0      0      0      0      0      0
Node 0, zone   Normal    304    540    116     13      2      2      0      0      0      0      0
=========================
             total       used       free     shared    buffers     cached
Mem:           497        487         10          0         63        303
-/+ buffers/cache:        120        376
Swap:         1458         34       1424
Total:        1956        522       1434
=========================
Total Memory Freed: 342 MB
Total Memory Freed: 53 MB
Total Memory Freed: 23 MB
Total Memory Freed: 10 MB
Total Memory Freed: 15 MB
Total Memory Freed: -1 MB
Node 0, zone      DMA      6      6      7      8     10      9      7      4      1      0      0
Node 0, zone   Normal   2129   2612   2166   1723   1260    759    359    108     10      0      0
=========================
             total       used       free     shared    buffers     cached
Mem:           497         47        449          0          0          5
-/+ buffers/cache:         41        455
Swap:         1458         97       1361
Total:        1956        145       1811
=========================

It was verified using a sample shell script "reclaim_memory.sh" which keeps recovering memory by doing "echo 500 > /dev/shrinkmem" until no further reclaim is possible.

The experiments were performed with various scenarios as follows:
a) Just after the boot up - (could recover around 150MB with 512MB RAM)
b) After running many applications include youtube videos, large tar files download - 

   [until free mem becomes < 10MB]
   [Could recover around 300MB in one shot]
c) Run reclaim, while download is in progress and video still playing - (Not applications killed)

d) revoke all background applications again, after running reclaim - (No impact, normal behavior)
   [Just it took little extra time to launch, as if it was launched for first time]


Please see more discussions on this in the last year mailing list:

https://lkml.org/lkml/2012/4/15/35 


Thank You!
With regards,
Pintu Kumar
Samsung - India

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