[KPhotoAlbum] More thumbnail investigations

Johannes Zarl-Zierl johannes at zarl-zierl.at
Sat May 19 14:53:04 BST 2018

Hi Robert,

Do you know how the improvements interact with high latency media (i.e. network mounts)?


Am 19. Mai 2018 05:24:41 MESZ schrieb Robert Krawitz <rlk at alum.mit.edu>:
>On Fri, 18 May 2018 19:11:14 -0400 (EDT), Robert Krawitz wrote:
>> On Fri, 18 May 2018 23:45:31 +0200, Johannes Zarl-Zierl wrote:
>>> Am Montag, 14. Mai 2018, 14:41:43 CEST schrieb Robert Krawitz:
>>>> The best solution would be to generate thumbnails upon image load
>>>> images up to a certain size.  That would combine nicely with the
>>>> code, which can also profit from having the entire file (since the
>>>> underlying crypto code in Qt only does 16K I/O ops).  We could
>>>> postpone the thumbnail generation for really big files (and files
>>>> need load methods other than JPEG or thumbnail extraction from RAW)
>>>> the end.
>>> My gut feeling is that we do read image files too many times (at
>>> least 3 times for exif, thumbnails, md5) and without optimizing for
>>> cache-friendliness.
>> We do indeed.  We may read them four times; I'm not certain.  I've
>> actually added a fourth one -- a scout thread (actually, I'm finding
>> that two scouts work best, but we can tune it) that slurps the data
>> into RAM, so the other reads are satisfied by buffering (and I've put
>> in protocol so the scout thread doesn't get too far ahead). 
>> thumbnail building with everything else helps too.
>So I did some more performance measurement, and found that one scout
>thread actually works best.  I also tuned the I/O sizes for both the
>scout thread and the MD5 checksumming.
>There turned out to be one more subtle (but quite significant)
>performance issue in the new image loading code; it was trying to
>compute MD5 checksums on all "modified" filenames, which can be
>expensive if you have a lot of suffix substitutions; it was on the
>order of 25-33% on both SSD and hard disk.  With that (and proper I/O
>tuning), I'm now getting the kind of I/O performance I expect on the
>hard disk (95 MB/sec or so with 100-110 IO/sec when reading 10 MB
>image files).  The hard disk maxes out around 115 MB/sec, but that
>needs sustained streaming I/O.  I'm getting about 350 MB/sec off the
>SSD, but that appears to be partly CPU limited on my system; if I turn
>off thumbnail building I get about 400-420 MB/sec (the peak is about
>550 MB/sec, and I've gotten close to that with sufficient threading).
>With an NVMe SSD I'd probably get a little better performance but not
>enough to matter.  Being able to load 10800 images in 4'30" is quite
>satisfactory (it's about 16'20" on hard disk).
>I'm pretty confident now (and I'm going to be preparing to push this
>code this weekend) that the image loading is pretty close to what
>we're going to get on a hard disk system; it would need some pretty
>fancy footwork to do better on an SSD.
>>>> This work may not be entirely trivial, but it could have a pretty
>>>> payoff when loading files.
>>> I've shied away from tackling this issue because of the complexity
>>> of the code it touches.
>> It's pretty complex code, to be sure, but this is the very first
>> people see (how fast does it read my photos, and how fast can I skim
>> through the thumbnails?), and if you have a lot of images, it's very
>> important from a workflow perspective.
>I'm going to try the thumbnail rebuild thing overnight; I'm curious
>whether some of my other changes are having a significant impact

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