Subject: Re: libpq compression
To: Robert Haas <robertmhaas@gmail.com>,
 Daniil Zakhlystov <usernamedt@yandex-team.ru>
Cc: Konstantin Knizhnik <k.knizhnik@postgrespro.ru>,
 Denis Smirnov <sd@arenadata.io>, Andrey Borodin <x4mmm@yandex-team.ru>,
 PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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From: Tomas Vondra <tomas.vondra@enterprisedb.com>
Message-ID: <d889e3a2-d3f4-3f8e-aa81-c142e32ddf57@enterprisedb.com>
Date: Tue, 22 Dec 2020 19:15:23 +0100
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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

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company