Re: SSL encryption makes bytea transfer slow

Heikki Linnakangas wrote:
>> We selected a 30MB bytea with psql connected with
>> "-h localhost" and found that it makes a huge
>> difference whether we have SSL encryption on or off.
>>
>> Without SSL the SELECT finished in about a second,
>> with SSL it took over 23 seconds (measured with
>> \timing in psql).
>> During that time, the CPU is 100% busy.
>> All data are cached in memory.
>>
>> Is this difference as expected?

Thanks for looking at that.

> I tried to reproduce that, but only saw about 4x difference in the
> timing, not 23x.

I tried more tests on an idle server, and the factor I observe here is
3 or 4 as you say.  The original measurements were taken on a server
under load.

> oprofile suggests that all that overhead is coming from compression.
> Apparently SSL does compression automatically. Oprofile report of the
> above test case with SSL enabled:
>
> samples  %        image name               symbol name
> 28177    74.4753  libz.so.1.2.3.4          /usr/lib/libz.so.1.2.3.4
> 1814      4.7946  postgres                 byteain
> 1459      3.8563  libc-2.13.so             __memcpy_ssse3_back
> 1437      3.7982  libcrypto.so.0.9.8       /usr/lib/libcrypto.so.0.9.8
> 896       2.3682  postgres                 hex_encode
> 304       0.8035  vmlinux-3.0.0-1-amd64    clear_page_c
> 271       0.7163  libc-2.13.so             __strlen_sse42
> 222       0.5868  libssl.so.0.9.8          /usr/lib/libssl.so.0.9.8
>
> And without:
>
> samples  %        image name               symbol name
> 1601     27.4144  postgres                 byteain
> 865      14.8116  postgres                 hex_encode
> 835      14.2979  libc-2.13.so             __memcpy_ssse3_back
> 290       4.9658  vmlinux-3.0.0-1-amd64    clear_page_c
> 280       4.7945  libc-2.13.so             __strlen_sse42
> 184       3.1507  vmlinux-3.0.0-1-amd64    page_fault
> 174       2.9795  vmlinux-3.0.0-1-amd64    put_mems_allowed
>
>
> Maybe your data is very expensive to compress for some reason?

Funny, I cannot see any calls to libz. On my system (RHEL 3, PostgreSQL
8.4.8,
openssl 0.9.7a) the oprofile reports of the server process look like
this:

With SSL:

samples  %           symbol name      image name
5326     77.6611     (no symbol)      /lib/libcrypto.so.0.9.7a
755      11.009      byteaout
/magwien/postgres-8.4.8/bin/postgres
378      5.51181     __GI_memcpy      /lib/tls/libc-2.3.2.so
220      3.20793     printtup
/magwien/postgres-8.4.8/bin/postgres

Without SSL:

samples  %           symbol name      image name
765      55.8394     byteaout
/magwien/postgres-8.4.8/bin/postgres
293      21.3869     __GI_memcpy      /lib/tls/libc-2.3.2.so
220      16.0584     printtup
/magwien/postgres-8.4.8/bin/postgres


Could that still be compression?

The test I am running is:

$ psql "host=localhost sslmode=... dbname=test"
test=> \o /dev/null
test=> select val from images where id=2;
test=> \q

Yours,
Laurenz Albe