On Fri, Aug 18, 2017 at 06:16:35PM -0700, Doug Porter wrote:
> Use a red-black tree to track allocations instead of a linked list.
> This provides a substantial performance boost when the number of
> allocations in a region is large. This change resulted in a reduction
> in runtime from 4821s to 6s on one filesystem.
>
> Signed-off-by: Doug Porter <dsp@xxxxxx>
Hi Doug, as it turns out, Jaco Kroon and I had been debugging the same
problem as oen you were working on. We came up with a different way
of solving this problem (see below). It works by observing that
unless the extent tree is terribly corrupted, region_allocate() will
always be appending to the very end of the list.
However, it has since occurred to me that since we are doing an
breadth-first traversal of the extent tree, the starting lba for each
leaf node *must* always be monotonically increasing, and we already
check to make sure this is true within an extent tree block. So I
think it might be possible to dispense with using any kind of data
structure, whether it's an rbtree or a linked list, and instead just
simply make sure we enforce the start+len of the last entry in an
extent tree block is < the starting lba of the first entry in the next
extent tree block.
We are already checking all of the necessary other conditions in
scan_extent_node, so with this additional check, I believe that using
the region tracking code in scan_extent_node (which was originally
written to make sure that extended attribute block did not have any
parts of a string shared between more than one EA key or value pair)
can be made entirely unnecessary for scan_extent_node().
I haven't had a chance to code this alternate fix, but I believe it
should be superior to either your patch or the one which I had drafted
below. Does this make sense to you?
- Ted
commit 8a48ce07a5923242fecc5dc04d6e30dd59a8f07d
Author: Theodore Ts'o <tytso@xxxxxxx>
Date: Mon Aug 14 19:52:39 2017 -0400
e2fsck: add optimization for large, fragmented sparse files
The code which checks for overlapping logical blocks in an extent tree
is O(h*e) in time, where h is the number of holes in the file, and e
is the number of extents in the file. So a file with a large number
of holes can take e2fsck a long time process. Optimize this taking
advantage of the fact the vast majority of the time, region_allocate()
is called with increasing logical block numbers, so we are almost
always append onto the end of the region list.
Signed-off-by: Theodore Ts'o <tytso@xxxxxxx>
diff --git a/e2fsck/region.c b/e2fsck/region.c
index e32f89db0..95d23be4f 100644
--- a/e2fsck/region.c
+++ b/e2fsck/region.c
@@ -30,6 +30,7 @@ struct region_struct {
region_addr_t min;
region_addr_t max;
struct region_el *allocated;
+ struct region_el *last;
};
region_t region_create(region_addr_t min, region_addr_t max)
@@ -42,6 +43,7 @@ region_t region_create(region_addr_t min, region_addr_t max)
memset(region, 0, sizeof(struct region_struct));
region->min = min;
region->max = max;
+ region->last = NULL;
return region;
}
@@ -68,6 +70,18 @@ int region_allocate(region_t region, region_addr_t start, int n)
if (n == 0)
return 1;
+ if (region->last && region->last->end == start &&
+ !region->last->next) {
+ region->last->end = end;
+ return 0;
+ }
+ if (region->last && start > region->last->end &&
+ !region->last->next) {
+ r = NULL;
+ prev = region->last;
+ goto append_to_list;
+ }
+
/*
* Search through the linked list. If we find that it
* conflicts witih something that's already allocated, return
@@ -92,6 +106,8 @@ int region_allocate(region_t region, region_addr_t start, int n)
r->end = next->end;
r->next = next->next;
free(next);
+ if (!r->next)
+ region->last = r;
return 0;
}
}
@@ -104,12 +120,15 @@ int region_allocate(region_t region, region_addr_t start, int n)
/*
* Insert a new region element structure into the linked list
*/
+append_to_list:
new_region = malloc(sizeof(struct region_el));
if (!new_region)
return -1;
new_region->start = start;
new_region->end = start + n;
new_region->next = r;
+ if (!new_region->next)
+ region->last = new_region;
if (prev)
prev->next = new_region;
else
