Some time ago, Ben Collins-Sussman shared with us some of the Google teams' findings about APR's memory subsystem[1]. In his mail, he says (among other things) the following:
Over at Google, we simply hacked APR to *never* hold on to blocks for recycling. Essentially, this makes apr_pool_destroy() always free() the block, and makes apr_pool_create() always call malloc() malloc. Poof, all the memory leak went away instantly. What was more troubling is that the use of the MaxMemFree directive -- which is supposed to limit the total size of the 'free memory' recycling list -- didn't seem to work for us. What we need to do is go back and debug this more carefully, and see if it's a bug in APR, apache, or just in our testing methodology. I came across this mail thread again recently as part of my own investigation into Apache's seeming ignorance of the MaxMemFree directive. So I poked Ben to see if he would/could share the patch to APR that they wound up with. He responded with the following (and the attached patch, which I had to reformat to get to a apply cleanly): From: Ben Collins-Sussman <suss...@google.com> Date: Mon, 14 Dec 2009 09:46:45 -0600 Message-ID: <944683b40912140746t7763f6f8i5af9c549a8b68...@mail.gmail.com> Subject: Re: Got Apache insta-free patch? Crazy simple patch. Basically, if memory_recycling is disabled, then we prevent any freed-up pool from *ever* be pushed on to the 'free list' of pools to recycle. The result is that all calls to apr_pool_destroy() causes the actual unix free() call to happen. (And likewise, because the 'free list' of recyclable pools is always empty, every call to apr_pool_create() causes malloc() to happen.) Why this works: apr's recycling logic is ridiculously primitive. If there are 50 pools on the 'free list' which are each 2K, and a request comes in for a new pool that needs 20k, *none of those recyclable pools* get recombined at all. They just sit there forever, while apr malloc's a new 20k pool. The result is that for really long-running processes, apr just gradually holds on to more and more memory over time, ever more fragmented. This isn't a problem with normal worker/slave mode, since slave httpd processes die and get respawned after N requests. But if you run in multithreaded mode (which is the default on Windows, I think), then it's just all one big process that runs forever, leaking forever. Why we feel this patch is fine for production: the whole apr pool system was written in the mid-90's, back when malloc() and free() were seriously expensive and made your app noticeably slower. The concept of recycling was seen as a way to prevent these expensive operations. These days there is *no* speed hit at all that we can see. Computers and OSes are just so much faster. Feel free to post this to the whole d...@subversion list if you wish. I'm "feeling free" to share this with y'all. I'm not vouching for its accuracy -- I haven't had a chance to test it myself yet. But it's Christmas time, so have a cup of good cheer and some more free() calls on Google. [1] http://svn.haxx.se/dev/archive-2008-10/0070.shtml -- C. Michael Pilato <cmpil...@collab.net> CollabNet <> www.collab.net <> Distributed Development On Demand
diff -ru srclib/apr/include/apr_pools.h srclib/apr/include/apr_pools.h
--- srclib/apr/include/apr_pools.h 2008-07-14 11:07:00.000000000 -0400
+++ srclib/apr/include/apr_pools.h 2009-12-14 10:52:16.000000000 -0500
@@ -765,6 +765,14 @@
#endif /* APR_POOL_DEBUG or DOXYGEN */
+
+/**
+ * Enable or disable recycling of memory in pools.
+ * @param enable Non-zero to enable recycling, 0 to disable it
+ */
+APR_DECLARE(void) apr_set_memory_recycling(int enable);
+
+
/** @} */
#ifdef __cplusplus
diff -ru srclib/apr/memory/unix/apr_pools.c srclib/apr/memory/unix/apr_pools.c
--- srclib/apr/memory/unix/apr_pools.c 2008-07-19 07:54:52.000000000 -0400
+++ srclib/apr/memory/unix/apr_pools.c 2009-12-14 10:53:06.000000000 -0500
@@ -40,6 +40,18 @@
#endif
+
+/*
+ * Runtime disabling of recycling
+ */
+
+static int memory_recycling = 1;
+
+void apr_set_memory_recycling(int enable)
+{
+ memory_recycling = enable;
+}
+
/*
* Magic numbers
*/
@@ -354,8 +366,9 @@
next = node->next;
index = node->index;
- if (max_free_index != APR_ALLOCATOR_MAX_FREE_UNLIMITED
- && index > current_free_index) {
+ if (!memory_recycling ||
+ (max_free_index != APR_ALLOCATOR_MAX_FREE_UNLIMITED
+ && index > current_free_index)) {
node->next = freelist;
freelist = node;
}
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