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File 0827-erts-Stop-marking-memory-regions-as-discardable-madv.patch of Package erlang (Revision b4f72044e1af8cfe32130ab849a401a1)

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From df450823fcdb1657743c94023318abe3731366bb Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?John=20H=C3=B6gberg?= <john@erlang.org>
Date: Fri, 17 Dec 2021 17:03:45 +0100
Subject: [PATCH] erts: Stop marking memory regions as discardable
 (`madvise(2)`)

This has caused a lot of annoying trouble over the years, and the
benefits never materialized: none of the supported OS's would not
reclaim any pages until they out of physical memory, and most users
already had some kind of overload protection that prevented that
from happening in the first place.
---
 erts/emulator/beam/erl_alloc_util.c | 177 ----------------------------
 erts/emulator/sys/common/erl_mmap.c |  19 ---
 erts/emulator/sys/common/erl_mmap.h |  64 ----------
 3 files changed, 260 deletions(-)

diff --git a/erts/emulator/beam/erl_alloc_util.c b/erts/emulator/beam/erl_alloc_util.c
index 727d2e5c3d..4315bb67ab 100644
--- a/erts/emulator/beam/erl_alloc_util.c
+++ b/erts/emulator/beam/erl_alloc_util.c
@@ -2538,155 +2538,9 @@ mbc_alloc(Allctr_t *allctr, Uint size)
     return BLK2UMEM(blk);
 }
 
-typedef struct {
-    char *ptr;
-    UWord size;
-} ErtsMemDiscardRegion;
-
-/* Construct a discard region for the user memory of a free block, letting the
- * OS reclaim its physical memory when required.
- *
- * Note that we're ignoring both the footer and everything that comes before
- * the minimum block size as the allocator uses those areas to manage the
- * block. */
-static void ERTS_INLINE
-mem_discard_start(Allctr_t *allocator, Block_t *block,
-                  ErtsMemDiscardRegion *out)
-{
-    UWord size = BLK_SZ(block);
-
-    ASSERT(size >= allocator->min_block_size);
-
-    if (size > (allocator->min_block_size + FBLK_FTR_SZ)) {
-        out->size = size - allocator->min_block_size - FBLK_FTR_SZ;
-    } else {
-        out->size = 0;
-    }
-
-    out->ptr = (char*)block + allocator->min_block_size;
-}
-
-/* Expands a discard region into a neighboring free block, allowing us to
- * discard the block header and first page.
- *
- * This is very important in small-allocation scenarios where no single block
- * is large enough to be discarded on its own. */
-static void ERTS_INLINE
-mem_discard_coalesce(Allctr_t *allocator, Block_t *neighbor,
-                     ErtsMemDiscardRegion *region)
-{
-    char *neighbor_start;
-
-    ASSERT(IS_FREE_BLK(neighbor));
-
-    neighbor_start = (char*)neighbor;
-
-    if (region->ptr >= neighbor_start) {
-        char *region_start_page;
-
-        region_start_page = region->ptr - SYS_PAGE_SIZE;
-        region_start_page = (char*)((UWord)region_start_page & ~SYS_PAGE_SZ_MASK);
-
-        /* Expand if our first page begins within the previous free block's
-         * unused data. */
-        if (region_start_page >= (neighbor_start + allocator->min_block_size)) {
-            region->size += (region->ptr - region_start_page) - FBLK_FTR_SZ;
-            region->ptr = region_start_page;
-        }
-    } else {
-        char *region_end_page;
-        UWord neighbor_size;
-
-        ASSERT(region->ptr <= neighbor_start);
-
-        region_end_page = region->ptr + region->size + SYS_PAGE_SIZE;
-        region_end_page = (char*)((UWord)region_end_page & ~SYS_PAGE_SZ_MASK);
-
-        neighbor_size = BLK_SZ(neighbor) - FBLK_FTR_SZ;
-
-        /* Expand if our last page ends anywhere within the next free block,
-         * sans the footer we'll inherit. */
-        if (region_end_page < neighbor_start + neighbor_size) {
-            region->size += region_end_page - (region->ptr + region->size);
-        }
-    }
-}
-
-static void ERTS_INLINE
-mem_discard_finish(Allctr_t *allocator, Block_t *block,
-                   ErtsMemDiscardRegion *region)
-{
-#ifdef DEBUG
-    char *block_start, *block_end;
-    UWord block_size;
-
-    block_size = BLK_SZ(block);
-
-    /* Ensure that the region is completely covered by the legal area of the
-     * free block. This must hold even when the region is too small to be
-     * discarded. */
-    if (region->size > 0) {
-        ASSERT(block_size > allocator->min_block_size + FBLK_FTR_SZ);
-
-        block_start = (char*)block + allocator->min_block_size;
-        block_end = (char*)block + block_size - FBLK_FTR_SZ;
-
-        ASSERT(region->size == 0 ||
-            (region->ptr + region->size <= block_end &&
-             region->ptr >= block_start &&
-             region->size <= block_size));
-    }
-#else
-    (void)allocator;
-    (void)block;
-#endif
-
-    if (region->size > SYS_PAGE_SIZE) {
-        UWord align_offset, size;
-        char *ptr;
-
-        align_offset = SYS_PAGE_SIZE - ((UWord)region->ptr & SYS_PAGE_SZ_MASK);
-
-        size = (region->size - align_offset) & ~SYS_PAGE_SZ_MASK;
-        ptr = region->ptr + align_offset;
-
-        if (size > 0) {
-            ASSERT(!((UWord)ptr & SYS_PAGE_SZ_MASK));
-            ASSERT(!(size & SYS_PAGE_SZ_MASK));
-
-            erts_mem_discard(ptr, size);
-        }
-    }
-}
-
-static void
-carrier_mem_discard_free_blocks(Allctr_t *allocator, Carrier_t *carrier)
-{
-    static const int MAX_BLOCKS_TO_DISCARD = 100;
-    Block_t *block;
-    int i;
-
-    block = allocator->first_fblk_in_mbc(allocator, carrier);
-    i = 0;
-
-    while (block != NULL && i < MAX_BLOCKS_TO_DISCARD) {
-        ErtsMemDiscardRegion region;
-
-        ASSERT(IS_FREE_BLK(block));
-
-        mem_discard_start(allocator, block, &region);
-        mem_discard_finish(allocator, block, &region);
-
-        block = allocator->next_fblk_in_mbc(allocator, carrier, block);
-        i++;
-    }
-}
-
 static void
 mbc_free(Allctr_t *allctr, ErtsAlcType_t type, void *p, Carrier_t **busy_pcrr_pp)
 {
-    ErtsMemDiscardRegion discard_region = {0};
-    int discard;
     Uint is_first_blk;
     Uint is_last_blk;
     Uint blk_sz;
@@ -2702,21 +2556,6 @@ mbc_free(Allctr_t *allctr, ErtsAlcType_t type, void *p, Carrier_t **busy_pcrr_pp
     ASSERT(IS_MBC_BLK(blk));
     ASSERT(blk_sz >= allctr->min_block_size);
 
-#ifndef DEBUG
-    /* We want to mark freed blocks as reclaimable to the OS, but it's a fairly
-     * expensive operation which doesn't do much good if we use it again soon
-     * after, so we limit it to deallocations on pooled carriers. */
-    discard = busy_pcrr_pp && *busy_pcrr_pp;
-#else
-    /* Always discard in debug mode, regardless of whether we're in the pool or
-     * not. */
-    discard = 1;
-#endif
-
-    if (discard) {
-        mem_discard_start(allctr, blk, &discard_region);
-    }
-
     HARD_CHECK_BLK_CARRIER(allctr, blk);
 
     crr = ABLK_TO_MBC(blk);
@@ -2734,10 +2573,6 @@ mbc_free(Allctr_t *allctr, ErtsAlcType_t type, void *p, Carrier_t **busy_pcrr_pp
 	blk = PREV_BLK(blk);
 	(*allctr->unlink_free_block)(allctr, blk);
 
-        if (discard) {
-            mem_discard_coalesce(allctr, blk, &discard_region);
-        }
-
 	blk_sz += MBC_FBLK_SZ(blk);
 	is_first_blk = IS_MBC_FIRST_FBLK(allctr, blk);
 	SET_MBC_FBLK_SZ(blk, blk_sz);
@@ -2754,10 +2589,6 @@ mbc_free(Allctr_t *allctr, ErtsAlcType_t type, void *p, Carrier_t **busy_pcrr_pp
 	    /* Coalesce with next block... */
 	    (*allctr->unlink_free_block)(allctr, nxt_blk);
 
-            if (discard) {
-                mem_discard_coalesce(allctr, nxt_blk, &discard_region);
-            }
-
 	    blk_sz += MBC_FBLK_SZ(nxt_blk);
 	    SET_MBC_FBLK_SZ(blk, blk_sz);
 
@@ -2794,10 +2625,6 @@ mbc_free(Allctr_t *allctr, ErtsAlcType_t type, void *p, Carrier_t **busy_pcrr_pp
 	(*allctr->link_free_block)(allctr, blk);
 	HARD_CHECK_BLK_CARRIER(allctr, blk);
 
-        if (discard) {
-            mem_discard_finish(allctr, blk, &discard_region);
-        }
-
         if (busy_pcrr_pp && *busy_pcrr_pp) {
             update_pooled_tree(allctr, crr, blk_sz);
         } else {
@@ -3942,12 +3769,8 @@ abandon_carrier(Allctr_t *allctr, Carrier_t *crr)
     unlink_carrier(&allctr->mbc_list, crr);
     allctr->remove_mbc(allctr, crr);
 
-    /* Mark our free blocks as unused and reclaimable to the OS. */
-    carrier_mem_discard_free_blocks(allctr, crr);
-
     cpool_insert(allctr, crr);
 
-
     iallctr = erts_atomic_read_nob(&crr->allctr);
     if (allctr == crr->cpool.orig_allctr) {
         /* preserve HOMECOMING flag */
-- 
2.31.1

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File 0827-erts-Stop-marking-memory-regions-as-discardable-madv.patch of Package erlang (Revision b4f72044e1af8cfe32130ab849a401a1)

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