I am currently trying to understand some fields from /proc/meminfo and am still a newbie at it.
With respect to the documentation,
"MemAvailable is an estimate of how much memory is available for starting new applications, without swapping. Calculated from MemFree, SReclaimable, the size of the file LRU lists, and the low watermarks in each zone. The estimate takes into account that the system needs some page cache to function well, and that not all reclaimable slab will be reclaimable, due to items being in use. The impact of those factors will vary from system to system."
Code:
long si_mem_available(void)
{
long available;
unsigned long pagecache;
unsigned long wmark_low = 0;
unsigned long pages[NR_LRU_LISTS];
unsigned long reclaimable;
struct zone *zone;
int lru;
for (lru = LRU_BASE; lru < NR_LRU_LISTS; lru++)
pages[lru] = global_node_page_state(NR_LRU_BASE + lru);
for_each_zone(zone)
wmark_low += low_wmark_pages(zone);
/*
* Estimate the amount of memory available for userspace allocations,
* without causing swapping or OOM.
*/
available = global_zone_page_state(NR_FREE_PAGES) - totalreserve_pages;
/*
* Not all the page cache can be freed, otherwise the system will
* start swapping or thrashing. Assume at least half of the page
* cache, or the low watermark worth of cache, needs to stay.
*/
pagecache = pages[LRU_ACTIVE_FILE] + pages[LRU_INACTIVE_FILE];
pagecache -= min(pagecache / 2, wmark_low);
available += pagecache;
/*
* Part of the reclaimable slab and other kernel memory consists of
* items that are in use, and cannot be freed. Cap this estimate at the
* low watermark.
*/
reclaimable = global_node_page_state_pages(NR_SLAB_RECLAIMABLE_B) +
global_node_page_state(NR_KERNEL_MISC_RECLAIMABLE);
available += reclaimable - min(reclaimable / 2, wmark_low);
if (available < 0)
available = 0;
return available;
}
During MemAvailable's calculation in the function si_mem_available(), we cap the reclaimable slab which should stay to least of half of reclaimable or the low watermark because of items in use. The remaining part can be freed.
Similar kind of capping is followed for pagecache.
My questions wrt this are:
1. What is the probability of this estimation of reclaimable/pagecache cap going wrong because of kernel not being able to free the part of reclaimable/pagecache which we are expecting to be freed?
2. What are the probable scenarios in which this could happen and what could be the implications?
3. Does MemAvailable's estimates have any further drawbacks that I should be aware of before trusting it?
