do not test a pool block space if we can not allocated from the block 4 times

src/core/ngx_palloc.c

diff --git a/src/core/ngx_palloc.c b/src/core/ngx_palloc.c
index a3f73f4..fd04768 100644
--- a/src/core/ngx_palloc.c
+++ b/src/core/ngx_palloc.c
@@ -25,6 +25,7 @@
     p->d.last = (u_char *) p + sizeof(ngx_pool_t);
     p->d.end = (u_char *) p + size;
     p->d.next = NULL;
+    p->d.failed = 0;
 
     size = size - sizeof(ngx_pool_t);
     p->max = (size < NGX_MAX_ALLOC_FROM_POOL) ? size : NGX_MAX_ALLOC_FROM_POOL;
@@ -189,6 +190,7 @@
 
     new->d.end = m + psize;
     new->d.next = NULL;
+    new->d.failed = 0;
 
     m += sizeof(ngx_pool_data_t);
     m = ngx_align_ptr(m, NGX_ALIGNMENT);
@@ -197,7 +199,7 @@
     current = pool->current;
 
     for (p = current; p->d.next; p = p->d.next) {
-        if ((size_t) (p->d.end - p->d.last) < NGX_ALIGNMENT) {
+        if (p->d.failed++ > 4) {
             current = p->d.next;
         }
     }

src/core/ngx_palloc.h

diff --git a/src/core/ngx_palloc.h b/src/core/ngx_palloc.h
index f9d5216..7706df1 100644
--- a/src/core/ngx_palloc.h
+++ b/src/core/ngx_palloc.h
@@ -46,6 +46,7 @@
     u_char               *last;
     u_char               *end;
     ngx_pool_t           *next;
+    ngx_uint_t            failed;
 } ngx_pool_data_t;