Re: Antw: Re: non-smooth progress indication for git fsck and git gc

Hi!

Here are some stats from the repository. First the fast import ones (which had
good performance, but probably all cached, also):

% git fast-import <../git-stream
/usr/lib/git/git-fast-import statistics:
---------------------------------------------------------------------
Alloc'd objects:      55000
Total objects:        51959 (        56 duplicates                  )
      blobs  :        47991 (         0 duplicates          0 deltas of       
  0 attempts)
      trees  :         3946 (        56 duplicates        994 deltas of      
3929 attempts)
      commits:           22 (         0 duplicates          0 deltas of       
  0 attempts)
      tags   :            0 (         0 duplicates          0 deltas of       
  0 attempts)
Total branches:          15 (        15 loads     )
      marks:        1048576 (     48013 unique    )
      atoms:          43335
Memory total:          9611 KiB
       pools:          7033 KiB
     objects:          2578 KiB
---------------------------------------------------------------------
pack_report: getpagesize()            =       4096
pack_report: core.packedGitWindowSize = 1073741824
pack_report: core.packedGitLimit      = 8589934592
pack_report: pack_used_ctr            =       1780
pack_report: pack_mmap_calls          =         23
pack_report: pack_open_windows        =          1 /          1
pack_report: pack_mapped              =    2905751 /    2905751
---------------------------------------------------------------------

Then the output from git-sizer:

Processing blobs: 47991                        
Processing trees: 3946                        
Processing commits: 22                        
Matching commits to trees: 22                        
Processing annotated tags: 0                        
Processing references: 15                        
| Name                         | Value     | Level of concern               |
| ---------------------------- | --------- | ------------------------------ |
| Overall repository size      |           |                                |
| * Blobs                      |           |                                |
|   * Total size               |  13.7 GiB | *                              |
|                              |           |                                |
| Biggest objects              |           |                                |
| * Trees                      |           |                                |
|   * Maximum entries      [1] |  13.4 k   | *************                  |
| * Blobs                      |           |                                |
|   * Maximum size         [2] |   279 MiB | *****************************  |
|                              |           |                                |
| Biggest checkouts            |           |                                |
| * Maximum path depth     [3] |    10     | *                              |
| * Maximum path length    [3] |   130 B   | *                              |
| * Total size of files    [3] |  8.63 GiB | *********                      |

[1]  b701345cbd4317276568b9d9890fd77f38933bdc
(refs/heads/master:Resources/default/CIFP)
[2]  19f54c4a7595667329c1be23200234f0fe50ac56
(refs/heads/master:Resources/default/apt.dat)
[3]  b0e3d3a2b7f2504117408f13486c905a8eb8fb1e (refs/heads/master^{tree})

Some notes:
[1] is a directory with many short (typically < 1kB) text files
[2] is a very large text file with many changes
[3] Yes, some paths are long

Regards,
Ulrich

quoted

quoted

Ævar Arnfjörð Bjarmason [off-list ref] schrieb am 20.08.2018 um 10:57

in
Nachricht [ref]:

On Mon, Aug 20 2018, Ulrich Windl wrote:

quoted

quoted

quoted

quoted

Jeff King [off-list ref] schrieb am 16.08.2018 um 22:55 in Nachricht

[ref]:

quoted

On Thu, Aug 16, 2018 at 10:35:53PM +0200, Ævar Arnfjörð Bjarmason wrote:

quoted

This is all interesting, but I think unrelated to what Ulrich is talking
about. Quote:

    Between the two phases of "git fsck" (checking directories and
    checking objects) there was a break of several seconds where no
    progress was indicated

I.e. it's not about the pause you get with your testcase (which is
certainly another issue) but the break between the two progress bars.

I think he's talking about both. What I said responds to this:

Hi guys!

Yes, I was wondering what git does between the two visible phases, and 

between

quoted

the lines I was suggesting another progress message between those phases.

At

quoted

least the maximum unspecific three-dot-message "Thinking..." could be 

displayed

quoted

;-) Of course anything more appropriate would be welcome.
Also that message should only be displayed if it's foreseeable that the
operation will take significant time. In my case (I just repeated it a few
minutes ago) the delay is significant (at least 10 seconds). As noted 

earlier I

quoted

was hoping to capture the timing in a screencast, but it seems all the 

delays

quoted

were just optimized away in the recording.

quoted

quoted

quoted

quoted

During "git gc" the writing objects phase did not update for some
seconds, but then the percentage counter jumped like from 15% to 42%.

But yeah, I missed that the fsck thing was specifically about a break
between two meters. That's a separate problem, but also worth
discussing (and hopefully much easier to address).

quoted

If you fsck this repository it'll take around (on my spinning rust
server) 30 seconds between 100% of "Checking object directories" before
you get any output from "Checking objects".

The breakdown of that is (this is from approximate eyeballing):

 * We spend 1-3 seconds just on this:

https://github.com/git/git/blob/63749b2dea5d1501ff85bab7b8a7f64911d21dea/pack

quoted

quoted

-check.c#L181

OK, so that's checking the sha1 over the .idx file. We could put a meter
on that. I wouldn't expect it to generally be all that slow outside of
pathological cases, since it scales with the number of objects (and 1s
is our minimum update anyway, so that might be OK as-is). Your case has
13M objects, which is quite large.

Sometimes an oldish CPU could bring performance surprises, maybe. Anyway

my

quoted

CPU is question is an AMD Phenom2 quad-core with 3.2GHz nominal, and there

is a

quoted

classic spinning disk with 5400RPM built in...

quoted

quoted

 * We spend the majority of the ~30s on this:

https://github.com/git/git/blob/63749b2dea5d1501ff85bab7b8a7f64911d21dea/pack

quoted

quoted

-check.c#L70-L79

This is hashing the actual packfile. This is potentially quite long,
especially if you have a ton of big objects.

That seems to apply. BTW: Is there a way go get some repository statistics
like a histogram of object sizes (or whatever that might be useful to help
making decisions)?

The git-sizer program is really helpful in this regard:
https://github.com/github/git-sizer 

quoted

quoted

I wonder if we need to do this as a separate step anyway, though. Our
verification is based on index-pack these days, which means it's going
to walk over the whole content as part of the "Indexing objects" step to
expand base objects and mark deltas for later. Could we feed this hash
as part of that walk over the data? It's not going to save us 30s, but
it's likely to be more efficient. And it would fold the effort naturally
into the existing progress meter.

quoted

 * Wes spend another 3-5 seconds on this QSORT:

https://github.com/git/git/blob/63749b2dea5d1501ff85bab7b8a7f64911d21dea/pack

quoted

quoted

-check.c#L105

That's a tough one. I'm not sure how we'd count it (how many compares we
do?). And each item is doing so little work that hitting the progress
code may make things noticeably slower.

If it's sorting, maybe add some code like (wild guess):

if (objects_to_sort > magic_number)
   message("Sorting something...");

I think a good solution to these cases is to just introduce something to
the progress.c mode where it learns how to display a counter where we
don't know what the end-state will be. Something like your proposed
magic_number can just be covered under the more general case where we
don't show the progress bar unless it's been 1 second (which I believe
is the default).

quoted

quoted

Again, your case is pretty big. Just based on the number of objects,
linux.git should be 1.5-2.5 seconds on your machine for the same
operation. Which I think may be small enough to ignore (or even just
print a generic before/after). It's really the 30s packfile hash that's
making the whole thing so terrible.

-Peff