From: Kenneth Marshall <ktm@rice.edu>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Daniil Zakhlystov <usernamedt@yandex-team.ru>
Cc: Konstantin Knizhnik <k.knizhnik@postgrespro.ru>
Cc: Denis Smirnov <sd@arenadata.io>
Cc: Andrey Borodin <x4mmm@yandex-team.ru>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: libpq compression
Date: Tue, 22 Dec 2020 12:23:16 -0600
Message-ID: <20201222182316.GG16082@aart.rice.edu> (raw)
In-Reply-To: <d889e3a2-d3f4-3f8e-aa81-c142e32ddf57@enterprisedb.com>
References: <6811D196-E2FB-40CC-B1C9-F19427FBF675@yandex-team.ru>
<CA+TgmoZMTLydjP3iWBir20OrSXTugxS2CXC1WP1nWM5=Zqz0Nw@mail.gmail.com>
<A1D0D9A4-1FC4-4311-83C7-EEA90749EE07@yandex-team.ru>
<CA+TgmoZrM9T_W9YR-vLYZXj-vC_+F-DER65A87+pDXRwUFjJXg@mail.gmail.com>
<D5354E7A-3B9F-4D32-B3AB-F65058D36500@yandex-team.ru>
<CCAB1F57-A71D-4DEC-9A9C-F325B44BAC29@arenadata.io>
<93027ee4-fefa-a8ca-2cc7-245c912bf824@postgrespro.ru>
<62527092-16BD-479F-B503-FA527AF3B0C2@yandex-team.ru>
<CA+TgmoZbVH4R5JA9_pyTSdnFTq-w2B1JvomeD+r0oo0xp1DPtQ@mail.gmail.com>
<d889e3a2-d3f4-3f8e-aa81-c142e32ddf57@enterprisedb.com>
On Tue, Dec 22, 2020 at 07:15:23PM +0100, Tomas Vondra wrote:
>
>
> On 12/22/20 6:56 PM, Robert Haas wrote:
> >On Tue, Dec 22, 2020 at 6:24 AM Daniil Zakhlystov
> ><usernamedt@yandex-team.ru> wrote:
> >>When using bidirectional compression, Postgres resource usage correlates with the selected compression level. For example, here is the Postgresql application memory usage:
> >>
> >>No compression - 1.2 GiB
> >>
> >>ZSTD
> >>zstd:1 - 1.4 GiB
> >>zstd:7 - 4.0 GiB
> >>zstd:13 - 17.7 GiB
> >>zstd:19 - 56.3 GiB
> >>zstd:20 - 109.8 GiB - did not succeed
> >>zstd:21, zstd:22 > 140 GiB
> >>Postgres process crashes (out of memory)
> >
> >Good grief. So, suppose we add compression and support zstd. Then, can
> >unprivileged user capable of connecting to the database can negotiate
> >for zstd level 1 and then choose to actually send data compressed at
> >zstd level 22, crashing the server if it doesn't have a crapton of
> >memory? Honestly, I wouldn't blame somebody for filing a CVE if we
> >allowed that sort of thing to happen. I'm not sure what the solution
> >is, but we can't leave a way for a malicious client to consume 140GB
> >of memory on the server *per connection*. I assumed decompression
> >memory was going to measured in kB or MB, not GB. Honestly, even at
> >say L7, if you've got max_connections=100 and a user who wants to make
> >trouble, you have a really big problem.
> >
> >Perhaps I'm being too pessimistic here, but man that's a lot of memory.
> >
>
> Maybe I'm just confused, but my assumption was this means there's a
> memory leak somewhere - that we're not resetting/freeing some piece
> of memory, or so. Why would zstd need so much memory? It seems like
> a pretty serious disadvantage, so how could it become so popular?
>
>
> regards
>
Hi,
It looks like the space needed for decompression is between 1kb and
3.75tb:
https://github.com/facebook/zstd/blob/dev/doc/zstd_compression_format.md#window_descriptor
Sheesh! Looks like it would definitely need to be bounded to control
resource use.
Regards,
Ken
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Cc: ktm@rice.edu, tomas.vondra@enterprisedb.com, robertmhaas@gmail.com, usernamedt@yandex-team.ru, k.knizhnik@postgrespro.ru, sd@arenadata.io, x4mmm@yandex-team.ru, pgsql-hackers@lists.postgresql.org
Subject: Re: libpq compression
In-Reply-To: <20201222182316.GG16082@aart.rice.edu>
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