extstore: experimental per-class free chunk limit

external comands only for the moment. allows specifying per-slab-class how
many chunks to leave free before causing flushing to storage.

the external page mover algo in previous commits has a few issues:

- relies too heavily on page mover. lots of constant activity under load.
- adjusting the item age level on the fly is too laggy, and can easily
  over-frefree or under-free. IE; class 3 has TTL 90, but class 4 has TTL 60
and most of the pages in memory, it won't free much until it lowers to 60.

Thinking this would be something like; % of total chunks in slab class.
easiest to set as a percentage of total memory or by write rate periodically.

from there TTL can be balanced almost as in the original algorithm; keep a
small global page pool for small items to allocate memory from, and pull
pages from or balance between storage-capable classes to align TTL.

1 files changed, 2 insertions, 3 deletions

diff --git a/storage.c b/storage.c
index dfb2025..1d4e41b 100644
--- a/storage.c
+++ b/storage.c
@@ -18,15 +18,14 @@ int lru_maintainer_store(void *storage, const int clsid) {
     int item_age = settings.ext_item_age;
     bool mem_limit_reached = false;
     unsigned int chunks_free;
-    unsigned int chunks_perslab;
     struct lru_pull_tail_return it_info;
     // FIXME: need to directly ask the slabber how big a class is
     if (slabs_clsid(settings.ext_item_size) > clsid)
         return 0;
     chunks_free = slabs_available_chunks(clsid, &mem_limit_reached,
-            NULL, &chunks_perslab);
+            NULL, NULL);
     // if we are low on chunks and no spare, push out early.
-    if (chunks_free < chunks_perslab && mem_limit_reached)
+    if (chunks_free < settings.ext_free_memchunks[clsid] && mem_limit_reached)
         item_age = 0;
 
     it_info.it = NULL;