Rich-T-kid commented on PR #10044:
URL: https://github.com/apache/arrow-rs/pull/10044#issuecomment-4623736500

   i'm suspecting the issue has to do with 
   ` let (client, server) = tokio::io::duplex(1024 * 1024);`
   per the [docs](https://docs.rs/tokio/latest/tokio/io/fn.duplex.html) "_The 
max_buf_size argument is the maximum amount of bytes that can be written to a 
side before the write returns Poll::Pending_." 
   
   The two regression cases both involve large variable-length data where the 
encoded payload can exceed 1MB:
   roundtrip/variable/8192x8 — 8 columns × 8192 rows ~1.96MB
   roundtrip/variable/65536x1 — 65536 rows, large values buffer
   
   This also shows up in the regression cases,
   `roundtrip/variable/8192x8     1.00  1251.7±29.66µs  1798.6 MB/sec    1.53  
1914.9±122.75µs  1175.7 MB/sec`
   `roundtrip/variable/65536x1    1.00  1285.3±51.72µs  1750.7 MB/sec    1.49  
1910.5±128.29µs  1177.8 MB/sec` 
   throughput falls flat.
   
   taking a look at the other benchmark results this seems consistant,
   `roundtrip/fixed/65536x8       1.00      2.2±0.03ms  1821.4 MB/sec    1.19   
   2.6±0.16ms  1532.6 MB/sec` throughput shrinks and as such causes more 
blocking to happen. 
   
   Even in the event where this isn't the reason for the slow down I think 1MB 
is still to small for realistic max throughput. 


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