Re: [PATCH 00/11] Add compression support to pstore

[Aruna Balakrishnaiah]

On Thursday 08 August 2013 09:38 AM, Aruna Balakrishnaiah wrote:
> Hi Tony,
>
> On Thursday 08 August 2013 03:52 AM, Tony Luck wrote:
> > On Tue, Aug 6, 2013 at 10:35 PM, Tony Luck <tony.luck@xxxxxxxxx> wrote:
> > > ERST is at the whim of the BIOS writer (the ACPI standard doesn't provide any
> > > suggestions on record sizes).  My systems support ~6K record size.
> > Off by a little - 7896 bytes on my current machine.
> >
> > > efivars has, IIRC, a 1k limit coded in the Linux back end.
> > My memory was correct for this one.
> >
> > Adding a little tracing to pstore_getrecords() I see this:
> >
> > pstore: inflated 3880 bytes compressed to 17459 bytes
> > pstore: inflated 2567 bytes compressed to 17531 bytes
> > pstore: inflated 4018 bytes compressed to 17488 bytes
> >
> > Which isn't at all what I expected.  The ERST backend
> > advertised a bufsize of 7896, and I have the default
> > kmsg_bytes of 10240.  So on my forced panic the code
> > decided to create a three part pstore dump.  The sum of
> > the pieces is close to, but a little over the target of 10K.
> > But I don't understand why the compressed sizes are so
> > much smaller that the ERST backend block size.
>
> The sizes of compressed text depends on the nature of uncompressed
> data that is captured from kmsg_dump, considering the worst
> case of plain text based on experiments 45% was thecompression achieved.
> So we chose a buffer of size psinfo->bufsize * 100/45.
> If the uncompressed data captured was more of plain text nature then it
> would take up size close to ERST backend block size. Thats the reason
> you see compressed data of 2.5k to 4.0k. 2.5k would have more
> repeated occurrences than 4.0k.
>
> The sum of 3 pstore records should not have exceeded kmsg_bytes.
> Is it after adding total_len in the fix patch? Will take a look at it.

The sum of first two records is less than kmsg_bytes, so it captures the 3rd record.
Only after 3rd record is captured and written total is evaluated against kmsg_bytes
when itexceeds the limit it stops capturing the next one.

This shall happen even without compression right? If total is checked before
write this can be avoided.

- Aruna

> > The uncompressed sizes appear to be close to constant.
> > The compression ratios vary from 14% to 23%
> >
> > Why do we get three small parts instead of two bigger
> > ones close the the 7896 ERST bufsize?
>
> Same explanation as given above.
>
> > -Tony
>
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