On Sat, Aug 11, 2018 at 05:47:56PM +0200, René Scharfe wrote:
> Object IDs to skip are stored in a shared static oid_array. Lookups do
> a binary search on the sorted array. The code checks if the object IDs
> are already in the correct order while loading and skips sorting in that
> case.
>
> Simplify the code by using an oidset instead. Memory usage is a bit
> higher, but lookups are done in constant time and there is no need to
> worry about any sort order.
>
> Embed the oidset into struct fsck_options to make its ownership clear
> (no hidden sharing) and avoid unnecessary pointer indirection.
I actually had a case[1] yesterday where it seems like oidset is a fair
bit slower than oid_array for a large set.
But:
- loading the skiplist into memory has pretty lousy performance
anyway. If we really care about performance of large lists, we
should define a sorted on-disk format that can be mmap'd and
searched directly. Or if people are willing to tolerate false
positives, even a bloom filter.
I've never really used a big skiplist myself, so I haven't done any
work towards those things.
- we could probably improve the speed of oidset. Two things I notice
about its implementation:
- it has to malloc for each entry, which I suspect is the main
bottleneck. We could probably pool-allocate blocks, and when
entries get removed just leave the allocations in place until we
clear(). Most callers tend to build up a set and then query it a
lot, or possibly remove items from the set until it's empty. But
my guess is that few or none want a long-lived set that they add
and remove from randomly.
- insertion lets you do check-and-insert as a single operation
(something I failed to notice in [1]). But it's not implemented
as efficiently as it could be, since the "contains" and "put"
operations do separate lookups. This doesn't matter for a set
that's queried a lot more, but for something like de-duping
(like I was doing in [1]) most operations are check-and-insert.
Most of that is just food for thought, but it possibly argues that we
should not care about performance characteristics for swapping out
oid_array and oidset here (i.e., that your patch is fine, and the
simplicity benefit is the most important thing).
[1] https://public-inbox.org/git/20180810232457.GG19875@xxxxxxxxxxxxxxxxxxxxx/
but note that it's buried pretty deep.
> ---
> fsck.c | 23 ++---------------------
> fsck.h | 8 +++++---
> 2 files changed, 7 insertions(+), 24 deletions(-)
Again, I didn't see anything wrong with the patch itself.
-Peff
